I am extremely interested in stellar evolution and its relevance to my field of study, variable stars. I read great things about this book before it was released and pre-ordered a copy from Amazon dot com. The book dealer I purchased it from sent me the wrong book. I reported the error via their website and they quickly refunded my credit card, but never sent me the correct book.
A few months later I was able to finally obtain a complimentary copy of the book from the publisher, and after a protracted wait, was anxious to dive into it. What I found was almost as disappointing as getting the wrong book. This isn't the book I thought I was getting either.
This book should have been named The Complete History of Stellar Astrophysics, or something equally boring but less misleading. The first several chapters are intended to give one an extensive amount of background knowledge so that if the author ever does actually begin to write about stars you will understand what he is saying. The chapters begin with Light and the Sun, Gravity and Motion, Atomic and Subatomic Particles, Transmutation of the Elements, What Makes the Sun Shine? and The Extended Solar Atmosphere. Are you bored yet? I am.
Finally, in chapter seven we are introduced to stars. The first section of this chapter, called Comparison of the Sun to Other Stars, is 7.1- Where and When Can the Stars Be Seen? Are you kidding me?!
It is now page 129 of 311 and he is now going to explain that if we go outside at night and look up...
I don't know if I will ever finish this book. It is a ridiculous way to tell a story, and the title is entirely misleading. It's like buying a book called "NASCAR Heroes of the 1990's" and beginning chapter one with the history of the internal combustion engine.
There are only 13 chapters in this book and the author has wasted my time reading seven chapters of background material to get me to the point of 'go out at night and look up, this is where you can see stars'. Chapter eight is finally about stars, The Lives of Stars. Maybe I'll skip ahead to that and see if it's worth going any further. But not today. I'm too annoyed.
Bottom line, if you want to read a text on the history of astrophysics, this is your book. If you want to read about stellar evolution skip the first seven chapters and refer back to them only if you need to understand some concept in more depth with the full history of the discovery process included.
In October 2011 the American Association of Variable Star Observers (AAVSO) held its centennial celebration in Boston at the Woburn Hilton. It was the culmination of 100 years of collecting and archiving variable star data for the scientific community, and for several staff members and me, the realization of almost two years of planning. By all accounts the centenary was a success and on the final night of the meeting I found myself sitting at a banquet table surrounded by friends and co-workers, nearly exhausted but very satisfied.
Traditionally, various awards are given out prior to the meal being served and this evening was no exception. I was very happy to have been responsible for recognizing one of the AAVSO’s most outstanding volunteers, Tom Bretl, that evening. Tom is a remarkable and conscientious worker who has become the most prolific member of the charts and sequences team. I was proud to have Tom and his wife sitting at the table with my wife and I.
Right after Tom had returned to the table from receiving his award, and the applause began to fade, AAVSO Director, Arne Henden, introduced John Toone from the British Astronomical Association. John started talking about the great strides made in the last ten years, standardizing variable star charts and sequences for observers around the world and how he and a few other key people had been largely responsible for the improvements and had set the standard for the next one hundred years. It was a story I was very familiar with. I had worked closely with John and a small group of people for years establishing guidelines for creating variable star comparison sequences and producing thousands of new charts for observers. And now as John was speaking it dawned on me that he was about to present an award for chart making and I didn’t know who the award was for. How could they do that? Why would they exclude me from a discussion about recognizing one of our team for their contributions?
As he began to rattle off the list of achievements and benchmarks I realized he was talking about me! Those dirty dogs had conspired to give me an award and managed to keep it a complete secret. I was stunned, surprised, embarrassed, proud, joyful and sad all at once. The emotions of the moment got the best of me and could feel myself beginning to cry. I asked Irene for some Kleenex and said, “Quickly, please… I can’t go up there crying in front of everyone!”
I barely remember the walk to the podium or the walk back to my seat. I do remember that I was unable to say anything when John presented me with the Third Charles Butterworth Award ever awarded by the Variable Star Section of the BAA. Arne even quipped, “in all these years I’ve never seen Mike at a loss for words,” much to the crowd’s delight.
The plaque itself is a remarkable work of art, and the thoughtfulness that went into designing it made it even more beautiful and personal. The front side is a replica of a variable star chart of my favorite variable star, IW Andromedae. It has the title, cardinal directions and border hand painted in silver on a black slate tablet. The stars are gemstones of varying sizes set in holes drilled to represent with remarkable accuracy the brightness of the stars in the field of view and the comparison stars in the sequence have been hand labeled in silver paint also. It is stunning.
|The original Third Charles Butterworth Award in 2011|
The back is hand engraved and reads, “This the third Charles Butterworth Award was presented to Mike Simonsen on 8th October 2011 by the Variable Star Section of the British Astronomical Association in recognition of his outstanding contribution to the development of charts and sequences.”
Another facet of this story is my personal ties to the other two recipients of the award. Gary Poyner, one of the leading visual observers in the world and a mentor and good friend was the first recipient and a few years later, another mentor and friend of mine, Arne Henden, who is also my boss and Director of the AAVSO, received the second Charles Butterworth Award.
I was honored and deeply moved to receive it, and I will always cherish this award, which makes the next part of this story very hard to tell.
A Thanksgiving tradition in our house is for Irene to run out on Thanksgiving morning and buy a copy of all the newspapers, which on this day each year are stuffed with circulars, catalogs and advertisements for sales that will begin the next morning on the biggest shopping day in America, Black Friday. Then she usually goes through them all page by page while I watch football on television, another Thanksgiving tradition here. I had made a good-sized turkey, mashed potatoes, stuffing and all the trimmings, and Irene’s father had joined us for the feast. Some time after the first game, when the dinner dishes had been put away, and before we began on round two of the feast, open-faced turkey sandwiches with gravy, I brought out the Butterworth plaque to show my father-in-law. He admired its craftsmanship and pretended to be interested as I explained all the details behind the graphics and citation, then we went back to watching football on the TV while Irene flipped though several hundred pages of advertisements.
I didn’t give the plaque another thought until a week later when I went to retrieve it from the living room coffee table where I had last seen it on Thanksgiving and it wasn’t there. I looked all over the house, even in places I knew it couldn’t be, just to know I had looked everywhere, and then a horrible sinking feeling came over me as I deduced what had most likely been the undignified end to my special hand crafted plaque. In the mass of newspapers and advertisements left on the table and strewn about the living room the Butterworth Award had gotten buried or mixed in with the mess and the week after Thanksgiving it had gone out to the curb with the trash. It was by now buried under several feet of garbage in a landfill somewhere never to be seen again. I cried out loud at the loss.
Irene felt bad, I was distraught and the Butterworth was gone. I had taken several close up detailed pictures of it, intending to put a picture of the front and back in a two-slot picture frame for my office in Cambridge, but the place of honor I had selected for the genuine article laid bare for a long time, before I covered it up with a poster and tried to forget the tragic loss. Eventually, I did hang the picture frame of the Butterworth in my office at AAVSO headquarters, but I didn’t tell anyone about the fate of the plaque. It simply hurt too much to talk about and I was too embarrassed to ask the BAA if there was any way I could have another one made. It became a sore spot for Irene and I also. I tried not to blame her for the loss, even though deep down I did hold her at least partially responsible, and she resented me blaming her for the loss when it was I who apparently threw it in the trash, not her. We went on like this for a year. When Thanksgiving came around in 2012 it dredged up all the feeling of loss all over again, and by mid-December I had gathered up enough courage and put aside my embarrassment to write John Toone and the BAAVSS Director, Roger Pickard, to ask if there was any way a replacement could be made. They replied within a day saying that a replica could be made and that it would cost approximately £75 plus postage and would take a couple months to make. I was ecstatic.
February 4, 2013 I received word the plaque was finished and ready to be shipped. I simply had to make arrangements to pay for the plaque and shipping costs, which I did that same day. It ended up costing me around $150.00 US, but it was worth every penny. I couldn’t wait for it to get here.
On February 8th the packaged arrived. I knew it was the plaque. It was a heavy cardboard envelope from the UK. What else could it be? I rushed up to the house from the mailbox and cut open the packaging. As I pulled the plaque from inside the envelope I could feel through the bubble wrap that something was not right. My heart sank. I peeled away the packing and to my utter dismay found the plaque broken into several pieces with shards and dust and little gemstones loose inside the bubble wrap. For the third time the Charles Butterworth Award caused a lump in my throat and made my eyes tear up.
|The second Third Charles Butterworth Award shattered|
I immediately notified Roger, John and Alan, the artisan who had made the first two plaques. I tried to glue the pieces together but nothing I had on hand worked on the natural stone. Alan replied a day later that the shipment had been insured and said that he could make yet another plaque and would ship it in a more robust package from the UK next time. I couldn’t imagine what could go wrong the next time, but I tried not to get my hopes up, since by this time I was feeling genuinely cursed.
March 18th I was in Cambridge for meetings and the upcoming DSLR Manual Workshop. At 5:30 PM I got a call on my cell phone, but didn’t pick it up because I was out to dinner with AAVSO Secretary, Gary Walker, and I don’t like to interrupt face-to-face conversations by answering the phone. When I got back to headquarters I saw I had a voice mail and there was an IM message from Irene saying, “I think the Butterworth plaque came in the mail!” So I called Irene. The phone rang several times before she answered, somewhat out of breath. “Is it the plaque?” I asked without even saying hello. “ I don’t know. I didn’t open it,” she said, still breathless.
“Can you open it?” I asked. “Right now?” she replied.
“I’m on the treadmill. Can I call you back later?”
Irene has her priorities, and I suppose she figured I had waited this long, another hour wasn’t going to kill me. So I waited.
About an hour later she called me back to tell me it was indeed the third Third Charles Butterworth Award and it was intact and looked great. She also said, “I put it in your sock drawer where it will be safe and I’m not touching it ever again after this.”
A week later when I got home from Boston I got to see the new plaque and it is indeed a handsome replica of the original. It came with a note from Alan, the artist who made them all, which read, “Dear Mr. Simonsen, I hope this made it to you in one piece. I have made it from slightly thicker slate and drilled a smaller hanging hole and used much better packing, so I am keeping my fingers crossed! Best wishes and apologies, Alan.”
I wrote back to Alan, Roger and John “The third "Third Charles Butterworth Award" has arrived safe and sound and will soon occupy a place of honor on the wall in my office.
Thank you very much, to all of you.”
Thank you very much, to all of you.”
|Leslie Peltier Award 2012, Charles Butterworth Award 2011 |
and the AAVSO Directors Award 2005
And so it is. The third Third Charles Butterworth Award now hangs in my office and will remain there until the stars fall from the sky.
And Black Friday will always have its own meaning to me.
Friday, February 15th, will be the 14th anniversary of my first variable star estimate.
R Leo, 9.6, Feb 15, 1999
Over 82,000 observations later, I can still recall a lot of things about my first variable star observation.
I remember I was at my dad’s house in Romeo, Michigan, where the skies were much darker than at my home at the time. I was using a 10” LX50 and finding objects by dialing in the setting circles and star hopping with the finder and a low power eyepiece. I had spent a couple months learning how to polar align, set up and tear down the telescope, and how to find things on star charts and use the telescope at different magnifications. Once I thought I knew my way around the telescope and the sky, I was determined to start variable star observing.
I tried for almost a whole night on the 14th, but I couldn’t figure out how to relate what I was seeing in the eyepiece to what I was seeing on the AAVSO charts. It was very cold that year in February. There were several inches of snow on the ground. I remember because I had lost my favorite pen in the snow in the dark, and spent a half an hour looking for it before giving up for the night, frustrated by my lack of success, the bitter cold and the loss of my pen.
But I was determined, so the next night I drove out to my father’s property, set up the telescope, polar aligned it and began looking for R Leo again. I had been trying to star hop from Regulus, heading west, looking for that little triangle of stars that everyone who observes R Leo comes to know so well. But I couldn’t tell what I was doing wrong or how to fix it, so I decided to try using the setting circles and a new finder chart I had made myself from a planetarium program called Mega Star.
Something I’ve learned over the years since then is this- if you make a mistake while trying to find a new star, and then get lost a second time, move on to another target and come back to it another night, because chances are you’re going to continue making the same mistake over and over. It happens to the best of us. Next time, you’ll wonder why you thought it was so tough the previous night, when armed with a fresh perspective, you land right on it.
That is what happened to me. Once I dropped the star hopping strategy and just dialed it in, I landed almost smack dab on top of it. I had probably slewed past it a dozen times the night before, but couldn’t tell how big the triangle I was looking for was going to be in my eyepiece or finder. When I was pointing right at it, undistracted, it hit me like a ton of bricks. There it was! And I was sure that was R Leo right there, because it was obviously redder than the other stars. I’m pretty sure I laughed out loud. I was relieved. “Hey, I can do this,” I said to myself.
A lot of things in my life have changed since that fateful night. The AAVSO has had an enormous impact on my life, more than I ever could have imagined standing in the snow that night in 1999.
It was years later that I learned several of my friends and legendary observer, Leslie Peltier, started their VSO careers observing R Leo. I go back and visit her now and then, just because R Leo will always have a special place in my heart. Some night this month the clouds have to part, so I can go back and relive a special moment in my memory one more time, with my oldest variable star friend.
|My personal history with R Leo. Blue crosses are my visual observations over the years.|
Today we're going to start something new on Simostronomy. I've invited a guest blogger, Gael Mariani, to add content to the stellar astronomy blog. Gael recently joined the AAVSO and has already contributed an excellent article to the AAVSO website on Henrietta Leavitt. This essay tells a story about one of my favorite astronomers, William Herschel, that I had never heard before. I am pleased to share it with you here.
Sir William Herschel, Variable Stars, Sunspots and the Price of Wheat
By Gael Mariani
Students and scholars of astronomy need little introduction to the life and work of the German-born English astronomer William Herschel (1738-1822). A true polymath, Herschel was a pioneer of the study of binary stars and nebulae, the discoverer of infrared radiation in sunlight, a skilled mathematician, optical lens grinder and telescope maker, a ground-breaking naturalist and a prolific classical composer. His discovery of the planet Uranus in 1781, as well as two of its moons and two more moons of Saturn, garnered him fame, acclaim and a place in astronomical history. However, not all of Herschel’s scientific work was equally well received, and not all his discoveries are as well known today.
One of Herschel’s key areas of study, and a subject of great fascination for him, was those stars that seemed to change their brightness: what we now call variable stars; and he was responsible for much of the progress made in the understanding of these distant suns. His son John Frederick W. Herschel wrote in the 1833 A Treatise on Astronomy that, thanks to his father’s catalogue of brightness of the stars in each constellation, ‘amateurs of the science with only good eyes, or moderate equipment, might employ their time to excellent advantage.’
In today’s science, we know why variable stars vary in brightness. But in Herschel’s time, this was still a source of some mystery. As he sought to understand why these stars appeared to change, he attempted to correlate the phenomenon with another that he had studied extensively, namely the existence of sunspots on our own planet’s nearest star. Herschel posed the hypothesis that these more distant suns might also possess spots, which perhaps were the cause of their vacillation from brightness to dimness. Just two centuries after Galileo had proposed that sunspots were dark clouds floating about in the solar atmosphere, Herschel shared the contemporary scientific view that the greater the number of spots on the sun, the more these would block out the light energy radiated to earth: hence, the ‘spottier’ a variable star, the less bright it would appear from Earth.
Spurred on by the fact that he had perfected a telescope that gave him a view of the sun whose clarity was unprecedented at the time, Herschel deepened his study of sunspots, and this led him to form a new and radical notion: the possibility of a correlation between the number of sunspots and Earth’s climate.
He had noticed that, between July 1795 and February 1800, there had been a number of days when there had been no sunspot activity at all. Then, they had suddenly returned in abundance. He wrote: ‘It appears to me . . . that our Sun has for some time past been labouring under a disposition, from which it is now in a fair way of recovering’. In 1801 he presented a paper to the Royal Society entitled ‘The Nature of the Sun’, in which he wrote: ‘I am now much inclined to believe that openings [sunspots] with great shallows, ridges, nodules and corrugations, instead of small indentation, may lead us to expect a copious emission of heat, and therefore mild seasons . . . A constant observation of the sun with this view, and a proper information respecting the general mildness or severity of the seasons, in all parts of the world, may bring this theory to perfection or refute it if it be not well founded.’
But how was Herschel to back up his hypothesis? Hampered by the lack of precise meteorological records by which to test his theory, he persevered by lateral thinking. Given the effects of lesser or greater quantities of sunshine on vegetation, it struck him that records of good or bad harvests might provide him with the data he needed. Any correlation between these and periods of many or few sunspots would theoretically support his argument. Using Adam Smith’s The Wealth of Nations as his source, he was able to single out five periods when, due to poor harvests, the price of wheat in England had been particularly high. Comparing these records to those of sunspot activity during those periods, he discovered to his surprise a clear correlation between poorer wheat harvests and a relative lack of sunspot activity. Contrary to what had been thought until then, the presence of sunspots did not reduce the amount of heat from the sun, the opposite was true: greater sunspot activity corresponded to good weather and lower wheat prices, while a lack of sunspots corresponded to high wheat prices, which implied less favourable weather. ‘It seems probable,’ he wrote, ‘that some temporary scarcity or defect of the vegetation has taken place when the sun has been without those appearances which we surmise to be the symptoms of a copious emission of light and heat’. As we now know, the sun emits greater ultraviolet radiation, causing more heating of the Earth’s atmosphere, during periods of greater sunspot activity, or solar maximum. But in Herschel’s time this was a revolutionary idea – and the apparent correlation with Earth’s climate made it more revolutionary still.
Excited by his findings, Herschel urged his scientific colleagues to examine solar activity in more detail. Sadly, far from praising his discovery, his peers responded with scepticism and even ridicule. A piece in The Edinburgh Review lambasted his ‘erroneous theory concerning the influence of the solar spots and the price of grain’ as a ‘grand absurdity’. Clearly, the world was not ready to accept such stuff. For once in his illustrious career, the great William Herschel had fallen flat and his attempt to wake the scientific community to his radical idea had failed.
And even to this day, the prevailing views remain largely unchanged. While nobody would now dispute the correlation between solar activity and geometric disturbances on Earth – one only has to think of the SOHO Satellite and the data it sends back, containing potential warnings of increases in solar activity which could have a detrimental effect on such things as telecommunications systems – scientists have generally remained deeply sceptical of claims that there may be a correlation between solar activity and weather on Earth. One respected meteorologist in the 1960s warned that climate researchers risked branding themselves as cranks if they entertained any notion of sun-weather relationships. And in the modern era of sensitive political debate over climate change and global warming, pointing at possible links between earthly weather and cycles of solar activity has become more charged and contentious than ever.
But the time may come when scientists will be forced to revise the orthodox view. Two hundred years after William Herschel urged the Fellows of the Royal Society to investigate the links between sunspots and Earth’s climate, Israeli scientists Dr Lev A Pustilnik and Dr Gregory Yom Din used modern statistical methods to re-examine Herschel’s ideas and concluded that the great astronomer had been right after all. The modern findings confirmed that wheat prices in England during that period did indeed fluctuate in line with solar activity, being higher at solar minimum than at solar maximum, suggesting that the crop was more difficult to grow when sunspot activity was at its lowest.
The implications of this finding go far deeper. In August 2012, scientists studying climate patterns in Central Europe, specifically the winter freezing patterns of the Rhine, revealed a striking correlation between unusually cold Central European winters and periods of low solar activity. The studies, headed by Frank Sirocko, Professor of Sedimentology and Paleoclimatology at the Institute of Geosciences of Johannes Gutenberg University, Mainz, Germany, suggest that the extremely cold European winters of 2010/11 were the result of the North Atlantic Oscillation which Sirocko and his team now link to the low solar activity during that time. Furthermore, the researchers found that out of fourteen episodes between 1780 and 1963 when, according to historical records, the Rhine is known to have frozen over, ten corresponded to periods of minimal sunspot activity – establishing for the first time a possible common link between very cold European winters of the last 230 years. The known 11-year cycle of solar activity makes it possible, according to these results, to predict to some degree how the number of sunspots at any given period could affect our climate on Earth. What first drew Professor Sirocko’s attention to this possibility was the fact that the 125-mile skating race he once attended in the Netherlands can only be held every 11 years, when the rivers freeze up. ‘There must be a reason for this,’ Sirocko remembers thinking, ‘and it turns out there is.’
The Sun Kings’, Stuart Clark, Princeton University Press, 2007
The Herschel Chronicle: The Life Story of William Herschel and his Sister Caroline Herschel, Constance Ann Lubbock, 1933
Understanding Variable Stars, Professor John R Percy, Cambridge University Press, 2007
Daily Science online article, August 2012
Several months ago I was invited by French AAVSO member, Laurent Corp, to give a talk for CAPAS 2012, a pro-am astronomy conference on double stars and variable stars in Rodez, France. I was happy to oblige and agreed to give a talk on 'Pulsating Stars in the AAVSO Program'. Neither the organizers or I could afford to fly me to France, so we agreed to do the presentation via Skype.
I sent them an advance copy of my PowerPoint which they translated into French. They would display the slides on two screens, one in English, the other in French, and on a third screen would be the live webcam shot of my cheery face, blown up to giant talking head dimensions. The early afternoon time slot I was assigned translated to 8:30AM local time on Saturday, September 29, the same day I was leaving to drive to Boston, via Buffalo, New York.
I logged into Skype around 8AM and began my talk around 8:40, as the talks before me had begun to run long and late. I essentially covered the history of observations of pulsating stars in the AAVSO, the types of pulsating stars we observe today, and some of the current questions in astrophysics related to stellar pulsation. I was able to stay on for a while after my talk to monitor some of the other speakers as they gave their talks, but eventually it was time to load my suitcase and computer bag in the car and head east towards New York.
Buffalo is a 7 hour drive from my home in Michigan, which happens to be almost exactly half-way between home and Boston. Realizing that the Patriots were playing the Buffalo Bills on Sunday, September 30, I had decided rather last minute to see if I could get a ticket to the game. I could drive to Buffalo on Saturday, go to the game on Sunday and drive the rest of the way to AAVSO HQ Monday morning. As luck would have it, a very good single seat ticket was available in a season ticket holder section down near the 20 yard line. I snatched it up and set the plan in motion.
The drive to Buffalo was uneventful, and I was just able to score a decent hotel room west of Buffalo for the night. I drove to Ralph Wilson Stadium early Sunday morning so I could check out the tailgate partying, pick up a souvenier and shoot some pictures. Deciding not to wear my Patriots hoodie, I opted to go incognito, disguised as a Buffalo fan. I purchased a crazy, fuzzy Bills hat to complete my disguise. Peaking into a VIP tent set up in the parking lot, I got to meet a couple of the Bill's cheerleaders, who obliged me with a picture. The Patriots won the game, coming from behind in the second half so everything worked out great!
I drove into Boston, early Monday morning and arrived shortly after noon. The next four days were filled with meetings from 8:30AM to 5:30PM and working at my desk and tallking on the phone until 11PM. We had some very productive meetings, discussing AAVSOnet, CHOICE, fundraising, grant writing, the variable star plotter, a new binocular program, and various other topics. When I left for home on Friday morning my to do list from those meetings was two pages long!
My next trip to Boston will be for the fall meeting. I'm sharing the drive with Kevin Paxson and Dan Taylor. I hope to see you there. Until next time...
October 17, 2012
|This artist’s impression shows the planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. Credit: ESO/L. Calçada/N. Risinger (skysurvey.org)|
One of the most exciting exoplanet discoveries in decades was announced yesterday in an online ESO press conference. The discovery was to be officially announced in an article in the journal Nature today, but due to the excitement surrounding this discovery ESO and Nature agreed to lift the embargo a day early. I think they realized they weren’t going to be able to keep the lid on it for another day. The fact that an Earth sized planet had been discovered orbiting one of our nearest stellar neighbors, Alpha Centauri B, was a headline dying to be exploited by the press.
The planet was detected using the HARPS instrument on the 3.6-meter telescope at ESO's La Silla Observatory in Chile. HARPS can measure the radial velocity of a star with extraordinary precision. A planet in orbit around a star causes the star to move towards and away from an observer on Earth. Due to the Doppler effect, this radial velocity change induces a redshift of the star's spectrum towards longer wavelengths as it moves away and a blueshift as it approaches. This tiny shift of the star's spectrum can be measured with a high-precision spectrograph such as HARPS and used to infer the presence of a planet. Of course it’s not quite as simple as that.
How Do They Do That?
Alpha Cen B is a spectral type K1V star only slightly less massive than our Sun and cooler. There are a lot of competing signals combined in the light from Alpha Cen B, inducing a radial-velocity “jitter”. In the process of filtering out these additional sources of noise the team of astronomers was able to learn quite a bit more about the star itself. They determined that the star has spots like our Sun. As a star rotates, spots will appear to move from one side of the stellar disk to the other, introducing a periodic signal. This will correspond to the rotational period of the star. The radial velocities of Alpha Centauri B show a clear signal at 38.7 days, the rotational period of the star. They also learned the star has a solar-like star spot cycle with activity increasing and then decreasing over the four year period of the observations.
Additionally, the team had to filter out the effects of the radial velocity changes due to the star being a member of a binary system, as well as the fact that on some occasions of poor seeing the light from Alpha Cen B was contaminated with light from the primary, Alpha Cen A. As if that weren’t enough, they also had to remove the effect of the changing velocity of the Earth in the direction of the star as it orbits the Sun before the signal of a small rocky planet orbiting the star could be detected.
“Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days,” says lead author of the paper, Xavier Dumusque (Geneva Observatory, Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal). “It’s an extraordinary discovery and it has pushed our technique to the limit!”
Why is this important?
The technical achievement alone makes this an extraordinary discovery. It is the lowest mass exoplanet ever discovered, and now the closest known. This is a major step forward in detecting Earth twins. Unfortunately, the planet orbits so close to its parent star (0.04 AU) that its surface temperature is estimated to be approximately 1500 degrees Kelvin, so the chance of the planet supporting any kind of life is doubtful. But, the precision required to obtain this result would also allow astronomers to detect a planet four times the mass of Earth in the habitable zone of a Sun-like star (habitable super-Earths) with periods in the range of 200 days.
|Alpha Cen Bb and the habitable zone around the host star. Credit: Greg Laughlin from EPO press conference|
The fact that this planet was discovered orbiting a star in the Alpha Centauri system sparks the imagination. How many science fiction books have speculated about the existence of planets around our nearest stellar neighbor? Now it is science fact. There is at least one planet in the Alpha Centauri system, and probably more.
"This is the first planet with a mass similar to Earth ever found around a star like the Sun. Its orbit is very close to its star and it must be much too hot for life as we know it," adds Stephane Udry (Geneva Observatory), a co-author of the paper and member of the team, "but it may well be just one planet in a system of several. Our other HARPS results, and new findings from Kepler, both show clearly that the majority of low-mass planets are found in such systems."
The Kepler mission has found 2300 candidate planets by searching for exoplanet transits among the 10,000 or more stars it monitors continuously. The majority of planet candidates detected by this transit method are very distant from us. In contrast, the planets found by HARPS are around stars close to the Sun, this new discovery being the closest yet. This makes them better targets for many kinds of additional follow-up observations such as characterizing the planet's atmosphere.
Astronomers will now continue with extensive Doppler monitoring of Alpha Centauri B to try to detect additional planets, perhaps some in the habitable zone. This will become increasingly difficult as the separation between the Alpha Cen binary is decreasing over the next several years. They may also try to observe it from space to see if they can detect the transit of the star across the face of Alpha Cen B. The eclipse will be too shallow to observe from the ground. There is about a 10% chance of success, with the odds being higher if the orbital plane is in line with the binary plane, estimated at 11% inclination with respect to Earth.
How does this fit into variable star science?
The study of variable stars is really the study of the secret lives of stars. How are they formed, how they live out their lives and what changes occur internally and externally as they evolve. We learn about the environments surrounding them, including planets and other companions, and their affect on these partners; and finally, how they end their lives slowly fading away, stripped of their atmospheres or violently exploding, seeding the universe with the materials to build more stars, planets and us.
At almost every phase in a star’s life it varies in its light output. If the variation is large enough and occurs on human timescales, we, the observers of the AAVSO, can record and study these changes, and we have now for over 100 years.
In that time we have learned about all kinds of variations in stellar output and how to interpret it. Some stars vary as they pulsate, actually changing size physically, growing and then shrinking again, sometimes with a precise period, sometimes irregularly. We’ve seen stars that appear to vary because star spots are transported across the face of the star as it rotates. We’ve witnessed stars being eclipsed by unseen companions in extremely close orbits around their center of gravity, and now we can see the incredibly small changes in the light of a star as a planet crosses in front of it from our point of view.
Alpha Centauri B exhibits all of these phenomena at the same time. It rotates, it pulsates, it has spots, it’s a member of a binary system, and now we know it has a planet, perhaps several, and there is a chance we can see them transit the face of our close stellar neighbor if we turn our satellites on them. It is becoming apparent that the more we look, the more we will find planets around stars everywhere. It has also become obvious that the closer we look, the more we will find every star is a variable star to one degree or another at one time or another in its life. Alpha Centauri B is another interesting and exciting member of the variable star zoo.