THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459316.18
2020-2021: CX
"Astronomy is the daughter of idleness."
Bernard le Bovier de Fontenelle (1657-1757)
THE DAILY ASTRONOMER
Tuesday, April 13, 2021
Exploratorium XL: And More Questions
Did you know you had a really embarrassing typo today (Monday. April 12, 2021)
-various
Yes....
How can accretion disks around black holes become hot enough to emit X-rays? Are black holes that hot? -S.C.
First, we should mention that accretion disks form when a black holes draws material away from a star that is gravitationally bound to it. The gaseous matter rotates around the black hole to form a disk. See image below:
Accretion disks become so heated they'll often emit X-rays, one of the most energetic types of electromagnetic radiation. The black hole does not heat this disk, at least not directly. Instead, the heating results from friction. Material closest to the black hole moves more quickly than that farther away. This differential rotation generates tremendous amounts of heat. The innermost matter can draw very close to the black hole's event horizon and so will attain relativistic velocities (speeds approaching that of light.) The frictional heating caused by the motion of this rapidly rotating gas against material rotating at a slower speed will release copious amounts of heat energy. Accretion disks that form around stars won't experience as much heating because the innermost gas can't move nearly as fast.
Black holes aren't hot, but the disks surrounding them certainly are. For this reason, astronomers can often infer the existence of black holes by observing powerful x-ray sources close to active stars.
You mentioned that stars generally form in clusters that eventually disperse. Have astronomers found any star that formed in the same cluster as the Sun?
-S. Franklin
First, some background: our Sun did not form alone. Four and a half billion years ago, when Sol developed from a gaseous nebula, it formed with many other stars that would ultimately become members of an open or galactic cluster. This cluster traveled along a circuit within the Milky Way Galaxy, following the arc that its primordial cloud described. Eventually, within less than a billion years of its formation, the cluster dispersed as its component stars moved slowly, but inexorably, away from its center and along individual trajectories through the spiral arms. As the Sun and its lost siblings have turned around the galaxy more than 20 times since their formation, the cluster components are now scattered far and wide through the expansive Milky Way Galaxy. The hope of many astronomers is to identify these lost siblings.
Considering how scattered our stellar sisters have become, would it be at all possible to find them? Well, astronomers have found one already: HD 162826, a star within the constellation Hercules. Although it is relatively close at a distance of 111 light years, HD 162826 is slightly fainter than the dimmest stars visible to the unaided eye.
The Sun's first identified sibling: HD 162826 Located within the constellation Hercules, HD 162826 was the first star positively identified as being one of the Sun's siblings, defined as the stars that formed with the Sun about 4.5 billion years ago. It is possible that the Sun might have more than a thousand "siblings" at different locations within the Milky Way Galaxy. At a distance of 111 light years, HD 162826 is likely the closest sibling.
Although astronomers have found one, the Sun's siblings might well number in the hundreds or even more than a thousand. How can those lost sisters ever be located? We know that in 2014 an astronomical research team from the University of Texas at Austin determined that HD 162826 was a sibling based both on its chemical composition and by tracing back its orbital trajectory as it moved around the galaxy. A gaseous cloud from which the Sun and its siblings arose had a consistent chemical composition so that each star that formed within it would be chemically similar. Astronomers can ascertain this chemical similarity through spectral analysis of these stars. In particular, they look for relative abundance of rare elements such as Barium. It is likely that propelled remnants from a supernova (explosion of a super massive star) both enriched the Sun's birth cloud and induced the gradual collapse precipitating the star formation. These remnants would contain a set amount of the heavier elements which the "metal-poor" cloud would have lacked prior the supernova particles' arrival. As a consequence, stars born out of a specific nebula would be stamped with a particular spectral signature that would, in theory, distinguish them from non-sibling stars.
The Galactic Archeology with Hermes (GALAH) survey aims to observe more than one million stars so as to determine their individual "DNA profile." The aim, in part, is to properly identify some of Sol's siblings: to know where the Sun's sisters have strayed. These observations will also lend astronomers some insights into the motions within the galaxy, itself: as leaves in a wind give us information about the direction and velocity of air streams. We don't know how many of the Sun's lost siblings we'll find through these observations. We do know, however, that they lurk out there somewhere.
Did the dinosaurs see Orion in their sky?
-W.G.
Nobody has ever asked me this excellent question before. The answer is a resounding no. Here, I am going to refer to the dinosaurs that lived up to the end of the Cretaceous Period 66 million years ago. Although constellations remain seemingly unchanged in Earth's skies for long periods of time, they do not persist over millions of years. First of all, the constellations will become distorted over tens of thousands of years due to proper stellar motions. Recall that all the stars, including the Sun, move at speeds often exceeding 100 miles per second. Over time, the stars comprising a given constellation will shift away from their positions as a result of their motions and that of our solar system.
Also, during the Mesozoic Era, the time period in which dinosaurs were dominant, the bright stars comprising Orion hadn't even been born yet. Betelgeuse, for instance, is only about 9 million years old! We should point out that the most massive stars have short life-spans, on the order of millions or tens of millions of years.
While it is true that the Pharaohs, the Stonhenge builders and the poets Ovid and Hesiod all saw Orion, the dinosaurs did not.
Just so I can plan, I need to know: how long will the Sun support life on Earth? I've heard five billion years, but also just one billion. How much time do we actually have? -Robert N.
At the moment, the Sun is fusing hydrogen into helium in its core. It has been doing so for five billion years and will continue to do so for the next five billion years, according to estimates. However, as the Sun ages, its luminosity (energy output) will gradually increase. As the luminosity increases, the solar constant (the amount of solar energy Earth receives) will also increase. In approximately 1.1 billion years from now, the Sun's luminosity will increase to such an extent that Earth will become too hot to sustain life. So, the Sun will continue to fuse hydrogen for the next five billion years, but we on Earth have about 1.1 billion years to go before the planet becomes inhospitable. Please plan accordingly....
Are the mythology prefaces over?
-L.C.
No, but intermittent.
We'll have one tomorrow.
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