On a seemingly rare clear night recently in Northeastern Ohio, Cuyahoga Astronomical Association (CAA) member, photographer Alan Studt traveled to Hinckley Lake for a bit of stargazing. CAA members can access Cleveland Metroparks for after-hours astronomy.
“Gale and I watched the nice ISS pass on Thursday night. Since it was clear Friday night we went to Hinckley Lake Reservation and sat by the lake while I shot a star trail. Nice surprise – the ISS flew by and photo-bombed the star trail!” — Alan Studt
Astronomers using ESO’s Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star, which has 700 times the diameter of the Sun. Each cell covers more than a quarter of the star’s diameter and measures about 120 million kilometers across. These new results are being published this week in the journal Nature.
Located 530 light-years from Earth in the constellation of Grus (The Crane), π1 Gruis is a cool red giant. It has about the same mass as our Sun, but is 700 times larger and several thousand times as bright. Our Sun will swell to become a similar red giant star in about five billion years.
An international team of astronomers led by Claudia Paladini (ESO) used the PIONIER instrument on European Southern Observatory’s (ESO’s) Very Large Telescope to observe π1 Gruis in greater detail than ever before. They found that the surface of this red giant has just a few convective cells, or granules, that are each about 120 million kilometers across — about a quarter of the star’s diameter. Just one of these granules would extend from the Sun to beyond Venus. The surfaces — known as photospheres — of many giant stars are obscured by dust, which hinders observations. However, in the case of π1 Gruis, although dust is present far from the star, it does not have a significant effect on the new infrared observations.
When π1 Gruis ran out of hydrogen to burn long ago, this ancient star ceased the first stage of its nuclear fusion program. It shrank as it ran out of energy, causing it to heat up to over 100 million degrees. These extreme temperatures fueled the star’s next phase as it began to fuse helium into heavier atoms such as carbon and oxygen. This intensely hot core then expelled the star’s outer layers, causing it to balloon to hundreds of times larger than its original size. The star we see today is a variable red giant. Until now, the surface of one of these stars has never before been imaged in detail.
By comparison, the Sun’s photosphere contains about two million convective cells, with typical diameters of just 1,500 kilometers. The vast size differences in the convective cells of these two stars can be explained in part by their varying surface gravities. π1 Gruis is just 1.5 times the mass of our Sun but much larger, resulting in a much lower surface gravity and just a few, extremely large, granules.
While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like this one gradually expel their outer layers, resulting in beautiful planetary nebulae. Previous studies of π1 Gruis found a shell of material 0.9 light-years away from the central star, thought to have been ejected around 20,000 years ago. This relatively short period in a star’s life lasts just a few tens of thousands of years – compared to the overall lifetime of several billion – and these observations reveal a new method for probing this fleeting red giant phase.
The July 2017 Membership Meeting of the Cuyahoga Astronomical Association (CAA) will take place Monday, July 10, at 7:30 PM.
Gary Kader, CAA member and Director of the Burrell Observatory of Baldwin Wallace University, will present — SEEING DOUBLE: FINDING AND OBSERVING DOUBLE STARS FOR FUN AND SCIENCE. About half the stars in the night sky are actually multiple stars. Some are visible as two stars in a telescope, those are visual binaries, while others are determined from analyzing their light. Many show contrasting colors, which make them fun to observe. Learn all about double stars in this interesting presentation.
CAA’s monthly meetings take place at the Rocky River Nature Center of the Cleveland Metroparks, 24000 Valley Parkway, North Olmsted. The program begins at 7:30 PM, followed by a social break which is followed, in turn, by a business meeting.
Non-members are welcome to attend the evening’s program!
The double star system VFTS 352 is located about 160,000 light-years away in the Tarantula Nebula. This remarkable region is the most active nursery of new stars in the nearby universe and new observations from ESO’s VLT have revealed that this pair of young stars is among the most extreme and strangest yet found.
VFTS 352 is composed of two very hot, bright and massive stars that orbit each other in little more than a day. The centers of the stars are separated by just 12 million kilometers. In fact, the stars are so close that their surfaces overlap and a bridge has formed between them. VFTS 352 is not only the most massive known in this tiny class of “overcontact binaries” — it has a combined mass of about 57 times that of the Sun — but it also contains the hottest components — with surface temperatures above 40,000 degrees Celsius.
Extreme stars like the two components of VFTS 352, play a key role in the evolution of galaxies and are thought to be the main producers of elements such as oxygen. Such double stars are also linked to exotic behavior such as that shown by “vampire stars,” where a smaller companion star sucks matter from the surface of its larger neighbor.
In the case of VFTS 352, however, both stars in the system are of almost identical size. Material is, therefore, not sucked from one to another, but instead may be shared. The component stars of VFTS 352 are estimated to be sharing about 30 percent of their material.
Such a system is very rare because this phase in the life of the stars is short, making it difficult to catch them in the act. Because the stars are so close together, astronomers think that strong tidal forces lead to enhanced mixing of the material in the stellar interiors.
“The VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing,” explains lead author Leonardo A. Almeida of the University of São Paulo, Brazil. “As such it’s a fascinating and important discovery.”
Astronomers predict that VFTS 352 will face a cataclysmic fate in one of two ways. The first potential outcome is the merging of the two stars, which would likely produce a rapidly rotating, and possibly magnetic, gigantic single star. “If it keeps spinning rapidly it might end its life in one of the most energetic explosions in the universe, known as a long-duration gamma-ray burst,” says the lead scientist of the project, Hugues Sana, of the University of Leuven in Belgium.
The second possibility is explained by the lead theoretical astrophysicist in the team, Selma de Mink of University of Amsterdam: “If the stars are mixed well enough, they both remain compact and the VFTS 352 system may avoid merging. This would lead the objects down a new evolutionary path that is completely different from classic stellar evolution predictions. In the case of VFTS 352, the components would likely end their lives in supernova explosions, forming a close binary system of black holes. Such a remarkable object would be an intense source of gravitational waves.”
Proving the existence of this second evolutionary path would be an observational breakthrough in the field of stellar astrophysics. Regardless of how VFTS 352 meets its demise, this system has already provided astronomers with valuable new insights into the poorly understood evolutionary processes of massive overcontact binary star systems.
The NASA/ESA Hubble Space Telescope has produced the most detailed image so far of Messier 9 (M9), a globular star cluster located close to the center of the galaxy. This ball of stars is too faint to see with the naked eye, yet Hubble can see over 250,000 individual stars shining in it.
M9, pictured here is a roughly spherical swarm of stars that lies around 25,000 light-years from Earth, near the center of the Milky Way, so close that the gravitational forces from the galactic center pull it slightly out of shape. Globular clusters are thought to harbor some of the oldest stars in our galaxy, born when the Universe was just a small fraction of its current age. As well as being far older than the Sun —around twice its age— the stars of M9 also have a markedly different composition, and are enriched with far fewer heavier elements than the Sun.
In particular, the elements crucial to life on Earth, like oxygen and carbon, and the iron that makes up our planet’s core, are very scarce in M9 and clusters like it. This is because the Universe’s heavier elements were gradually formed in the cores of stars, and in supernova explosions. When the stars of M9 formed, there were far smaller quantities of these elements in existence.
M9, as its name suggests, was discovered by the great French comet hunter Charles Messier in 1764. Even through the most advanced telescopes of the day, none of the stars in the cluster could be seen individually. Messier, seeing only a faint smudge, therefore classified the object as a nebula –or “cloud” in Latin– and put it on his list of objects that looked like but were not comets. It was only later in the 18th century that astronomers, most notably William Herschel, began to spot stars within the cluster.
The contrast between Messier’s equipment and the tools at the disposal of today’s astronomers is stark. Hubble’s image, the highest resolution image yet made of M9, is able to resolve individual stars, right into the crowded center of the cluster. Over 250,000 of them are neatly focused on the detector of Hubble’s Advanced Camera for Surveys, in an image which covers an area of sky no bigger than the size of the head of a pin held at arm’s length.
As well as showing the individual stars, Hubble’s image clearly shows the different colors of the stars. A star’s color is directly related to its temperature — counter-intuitively, perhaps, the redder it is, the cooler it is; and the bluer it is, the hotter. The wide range of stellar temperatures here is clearly displayed by the broad palette of colors visible in Hubble’s image of M9.
During the summer months, stargazers can see the famous Summer Triangle almost directly overhead. The triangle is a giant asterism created by drawing imaginary lines between three bright stars — Deneb in the constellation Cygnus, Vega in the constellation Lyra, and Altair in the constellation Aquila.
Deneb is a blue-white super-giant that is almost 200 times larger than the Sun and 60,000 times brighter. At 1,500 light years distant, it is one of the most luminous stars known and is the farthest first-magnitude star from Earth. It has a solar wind that is 100,000 times faster than the solar wind from the Sun.
Vega is a blue-tinged white star that is about 25 light years away. It is twice the mass of the Sun and about 37 times brighter. At 16,000-degrees F, its surface temperature is almost twice as hot as the surface of the Sun. With the exception of the Sun, Vega is the first star to have been photographed. In 14,000 years, it will replace Polaris as our north star.
Altair, at a distance of 16.9 light years is about 1.5 times larger than the Sun, and is one of the closest stars visible to the naked eye. Altair spins on its axis at about 640,000 mph and completes a full revolution every 6.5 hours. The Sun in comparison takes about 25 days to complete one revolution, as measured at its equator. Altair is spinning so fast that its north and south poles are pushed in, giving the star an oblate appearance.
The Summer Triangle was first described as a triangle by Austrian astronomer J.J. Littrow in his atlas in 1866. German astronomer Johann Bode connected the stars in a map in 1816, but did not label the asterism.