THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet
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70 Falmouth Street Portland, Maine 04103 43.6667° N
70.2667° W Altitude: 10 feet below sea level 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:
[image: art_blackhole_accretion_star.jpg]
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.
[image: 1280px-HD162826-starmap.png]
*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.
[image: orion_constellation.jpg]
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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