Post by: Captain Foresight on 12/07/2022 18:37:46
Post by: Kryptid on 12/07/2022 20:44:43
Post by: Colin2B on 12/07/2022 23:22:48
Considering the universe is constantly expanding, why do nebula and stars contained therein have what would appear to be the same shape as viewed by the” ancient Greeks”The star constellations do not have the same shape now as in previous era. It’s not to do with expansion but relative drift between different systems. There are some astronomy apps that will show you the changes over time, but don’t expect dramatic shifts.
Post by: evan_au on 13/07/2022 02:27:00
I heard that some researchers had extrapolated stellar proper motions backward in time to see what the constellations would have looked like thousands of years ago. They came up with some date that they thought the familiar Ancient Greek constellations looked "most like" their name. From this they deduced a date when these constellations may have been originally named.
- I've not heard of the same thing being done for Australian Aboriginal constellations, which are apparently more about galactic dust clouds, and it would be much harder to measure a proper motion (compared to an individual star)..
Of course there is another explanation that I heard, about some Ancient Greek astronomers spending the night outside, with a very large amphora of ouzo...
Post by: Bored chemist on 13/07/2022 09:08:18
Of course there is another explanation that I heard, about some Ancient Greek astronomers spending the night outside, with a very large amphora of ouzoI'm fairly sure that the names pre-date the discovery of distillation.
So they must have been drinking something else.
When was retsina invented?
Post by: Janus on 13/07/2022 16:35:04
Considering the universe is constantly expanding, why do nebula and stars contained therein have what would appear to be the same shape as viewed by the” ancient Greeks”Gravitational attraction overcomes the expansion until fairly large scales. Anything smaller than a galaxy cluster is bound together by gravity and resists expansion. So when they say the universe is expanding, they mean overall, but not locally. Galaxy clusters move apart, but the galaxies within each cluster don't.
When you get down to smaller scales, like our local group of galaxies, you can even have galaxies on (slow motion) collision courses. The Andromeda galaxy, for example, will(possibly*) collide with ours in 4.5 billion years. This means that in the last 10,000 years it has only decreased its distance to us by 0.0002222... % Not exactly something noticeable by even the keenest observer.
*We don't have an accurate enough measure of its exact direction of travel to say for sure if it will be a collision or a very near miss.
Post by: SeanB on 14/07/2022 07:22:08
Post by: CatherineMaguire on 19/07/2022 17:32:30
Post by: evan_au on 20/07/2022 00:26:16
Quote from: CatherineMaguire
It is a well known fact that stars that we see don't exist anymoreIt is a well known expectation that most of the stars we see with our eyes do still exist.
- There are a few exceptions, such as a few stars that are thought to be nearing the end of their lives, and displaying some instability. They are very bright, so we see them from far away, and it is possible that they have already gone supernova.
- But most of the stars we can see have life expectancies far in excess of the light-travel time.
Post by: CatherineMaguire on 20/07/2022 08:26:22
Ok, I agree, but there are more theories and, anyway, the most spread away is that weQuote from: CatherineMaguireIt is a well known fact that stars that we see don't exist anymoreIt is a well known expectation that most of the stars we see with our eyes do still exist.
see
just the stars' light which don't exist anymore
Post by: Origin on 20/07/2022 14:00:00
just the stars' light which don't exist anymoreNot really. For example light (photons) from the sun that don't hit anything will essentially exist for ever, they will just be red shifted from the expansion of space.
Post by: evan_au on 20/07/2022 22:34:55
Quote from: me
a few stars that are thought to be nearing the end of their livesHere is a "Top 10" list of stars thought to be ready to "pop their top", along with a short bio of each...
For many of them, light takes over 1,000 years to reach us,
https://naturenoon.com/next-star-supernova/
Here is a more technical list (with a lot less description):
https://en.wikipedia.org/wiki/List_of_supernova_candidates
Astronomers expect 1 supernova in our galaxy about every 50 years, but historical records suggest that we only can see a naked-eye supernova about every 400 years (theories for the discrepancy vary...).
Overall, out of the roughly 7,000 individual stars visible in the night sky, probably all of them will still be visible next year.
Post by: Deecart on 20/07/2022 23:49:52
Quote from: CatherineMaguire
It is a well known fact that stars that we see don't exist anymore, the distance of light is blamed in the consequence that we see them now
At some point you are right.
If you consider that what you name "stars" are in fact galaxies or clusters of galaxies, it is true that, accordingly to the big bang therory and the expansion that come with it, already 97% of galaxies are actualy, from our point of view, unreachable.
Because 97% of all the galaxies inside the observable universe are recessing faster then the speed of light.
At some point, i think that we can say that physicaly speaking they do not exists for us any more.
But many "bright points" in the sky are also stars, real stars, because we are in our galaxy and we speak then of the stars of our galaxy.
There are 100 billions of visible galaxies (reachable or not) and 100 billions of stars in our galaxy (or 400 billions but this is around this).
So let say half of the "bright points" are stars and half of them are galaxies (
galaxies and same for stars)Post by: Halc on 21/07/2022 01:35:24
Hence the list is limited to about 2500 stars (7000 according to @evan_au who apparently lives in a unlikely dark place), the vast majority of which likely still exist today.
The list of potential stars that are exceptions were all particularly nearby and bright things, suggesting that there's a far more boring list of similar candidates that are just barely visible due to their far greater distance. But this greater distance also increases the window in which it can cease to exist. We see very few dim stars like our own, and most of them are fairly large bright distant stars.
We see at least one galaxy (I can still spot it here without aid despite the light pollution), but no individual stars in it are visible. The galaxy is assuredly still there, so I don't think it counts as something that has self-destructed in the time it took for the light to get here.
If you consider that what you name "stars" are in fact galaxies or clusters of galaxies, it is true that, accordingly to the big bang therory and the expansion that come with it, already 97% of galaxies are actualy, from our point of view, unreachable.Having a recession speed greater than light isn't what prevents us from getting to most of them. We see them, and they see us, so clearly something is 'getting to them'. What prevents us from getting to them is being beyond their event horizon, and while that event horizon and the distance where things have a recession rate (measured in cosmological coordinates) greater than c is close to it, it is inside, and we can theoretically reach something currently receding at a rate greater than c. Just not much faster.
Because 97% of all the galaxies inside the observable universe are recessing faster then the speed of light.
In the absence of acceleration of expansion, there would be no point in the universe that is unreachable regardless of recession rate. But there is acceleration, and that means that our local group of galaxies will never reach another group. Our current neighbors are all we will ever have.
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At some point, i think that we can say that physicaly speaking they do not exists for us any more.If you can see it, it physically exists (at least with a measurement definition of 'exists', and I don't know of a better definition), and over time, new objects come into the visible universe. They cannot leave it by definition, so over time, more and more stuff exists to us, despite all of it eventually fading to red to the point of undetectability. There is a limit beyond which we will never see even in infinite time, and currently about 60% of that is within our visible universe, and the other 40% will move in over time. Everything else will never 'exist' in any meaningful way.
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There are 100 billions of visible galaxies (reachable or not) and 100 billions of stars in our galaxy (or 400 billions but this is around this).I think, given a good telescope, that there are far more galaxies visible than local (in our galaxy) stars. The deep field shots you see are all galaxies and no stars. The shots looking into our own galaxy find it impossible to see through the dust and other material obscuring the view. The vast majority of stars are not visible, but the galaxies are unless they happen to lie on the same plane where all the obscured stars are. I am unsure of the percentage of the deep sky that is available for unobscured viewing.
So let say half of the "bright points" are stars and half of them are galaxies (
News: Earlier this year they broke the record for most distant object (called SD1), with a redshift of ~13¼, breaking the old record of about 11. The JWST did not get credit for this find. SD1 is young, small and bright, but I suspect it very much still exists (certainly as part of something bigger just like us) today.
Post by: Deecart on 21/07/2022 02:34:57
Fisrt point =>
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Although single stars from other galaxies can’t be seen with ours eye alone, some galaxies and nebula can be. For example, the Andromeda galaxy is a collection of over 1 trillion stars approximately 2.5 million light-years away. It can be seen as a fuzzy oval in the night sky, although you need a clear night in a rural area to see it.https://www.spaceanswers.com/solar-system/could-a-human-survive-on-the-surface-of-mercury/
You can easily do confusion between stars and galaxies.
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Today, when talking about stars and deep sky objects being ‘in’ a given constellation, astronomers mean to say that they lie within the defined boundaries of the constellation. Constellations themselves are not real, since the stars and deep sky objects (nebulae, galaxies, clusters) belonging to them lie at very different distances from Earth and only appear close to one another because they lie in the same line of sight when seen from Earth.https://www.constellation-guide.com/what-is-a-constellation/
Since the stars and other visible objects are located at different distances, this also means that we are seeing them as they were at very different points in the past and not as they appear now. Antares in Scorpius, for instance, lies about 550 light years from Earth. We are therefore seeing it as it appeared some 550 years ago. The Trifid Nebula in Sagittarius is approximately 4,100 light years distant, so the image we are seeing is 4,100 years old. Galaxies lie at even greater distances. The famous Antennae Galaxies in Corvus are located some 45 million light years from Earth. This makes the images of the colliding pair 45 million years old.
Post by: evan_au on 21/07/2022 10:48:30
Quote from: Halc
The deep field shots you see are all galaxies and no starsThere are a few nearby stars in any significant area of the sky.
The JWST Deep Field released last week was said to cover a fraction of the sky equivalent to a grain of sand held at arm's length.
- But even in this small fraction of the sky there were some individual stars, recognizable by the 6 radiating "spokes" - these are diffraction effects from the hexagonal mirrors, and the three supports for the prime focus mirror.
- This is different from galaxies, which tend to be fainter and fuzzier. Because they are fainter, the diffraction effects aren't so noticeable.
- Stars in Hubble images have "cross hairs" from the optical support structures inside Hubble, which are at right-angles.
- Looking closer at the JWST image, the stars also have some less prominent horizontal lines; I assume that these are from some other part of the optical path which has horizontal and vertical supports (like Hubble)
Webb_First_Deep_Field_NASA.png (1222.29 kB . 1462x896 - viewed 2431 times)
Post by: JesWade21 on 16/08/2022 12:33:37
Post by: Origin on 16/08/2022 14:43:36
The colour of a star is directly related to its brightness.I don't believe this is correct. The color of a star is directly related to it's temperature not brightness. For instance a red dwarf and a red giant with the same color will have the same surface temperature but the red giant will be much brighter.
Post by: evan_au on 16/08/2022 23:26:01
Quote from: JesWade21
We use parallax for nearby stars... (for) distant stars (further than 400 light years)The Gaia spacecraft has taken parallax measurement to a whole new level, with unparalleled precision in plotting star positions on the sky.
In conjunction with ground-based Doppler measurements, it has measured the 3-D position and velocity of a billion stars in our own galaxy, and even stars in the the Large Magellenic Cloud dwarf galaxy (150,000 light-years away).
https://en.wikipedia.org/wiki/Gaia_(spacecraft) (https://en.wikipedia.org/wiki/Gaia_(spacecraft))
Post by: Bored chemist on 17/08/2022 07:38:11
The apparent brightness is related to temperature, distance and size.The colour of a star is directly related to its brightness.I don't believe this is correct. The color of a star is directly related to it's temperature not brightness. For instance a red dwarf and a red giant with the same color will have the same surface temperature but the red giant will be much brighter.
The colour is related to temperature.
It might be possible to disentangle them.
Post by: evan_au on 17/08/2022 10:34:08
Quote from: bored chemist
It might be possible to disentangle them.Astronomers have lots of experience estimating the type of star.
Knowing the distance (eg with Gaia data) removes one of the unknowns.
Knowing the spectrum of a star allows you to determine the chemical composition, which gives some hints about whether it is a nearby red dwarf (still young in a lifetime that could last > 10 billion years), or a distant red giant (exhausted the hydrogen or helium fuel in its core), and soon to expire dramatically
- The spectrum also allows you to determine the temperature more accurately than just the colour, as some spectral lines are more prominent in certain temperature ranges.
Looking at the brightness variation over time also shows that some stars are entering a period of oscillation, which tells something of the internal structure of the star.
For some stars, we see transiting planets, which can give an idea of the star's mass. In future, we may be able to do this for more stars, using a coronagraph to image planets that don't pass directly in front of the star.
Post by: Janus on 17/08/2022 16:07:58
Quote from: bored chemistIt might be possible to disentangle them.
- The spectrum also allows you to determine the temperature more accurately than just the colour, as some spectral lines are more prominent in certain temperature ranges.
There is also spectral line spreading. Since the atoms of the star's surface are moving in random directions, the light coming from them will show a small varation of Doppler shifts. The hotter the surface, the greater the range. This leads to a slight blurring of the spectral lines, with cooler stars blurred less, and hotter stars blurred more.
Post by: Bored chemist on 17/08/2022 19:22:31
In some cases , we can do slightly better.Quote from: bored chemistIt might be possible to disentangle them.
- The spectrum also allows you to determine the temperature more accurately than just the colour, as some spectral lines are more prominent in certain temperature ranges.
There is also spectral line spreading. Since the atoms of the star's surface are moving in random directions, the light coming from them will show a small varation of Doppler shifts. The hotter the surface, the greater the range. This leads to a slight blurring of the spectral lines, with cooler stars blurred less, and hotter stars blurred more.
Superimposed on eth broadening, there's the fact that most stars spin (from our point of view). So one side is moving away from is and the other is moving towards us.
That doppler shifting can sometimes be seen too.
Post by: Petrochemicals on 17/08/2022 22:49:02
Post by: evan_au on 18/08/2022 00:01:12
Quote from: bored chemist
one side is moving away from is and the other is moving towards us.The NICER X-Ray telescope on the ISS studies the doppler shift of rotating neutron stars, in order to determine the mass vs diameter relationship, with the goal of better understanding the state diagram of neutronium.
https://en.wikipedia.org/wiki/Neutron_Star_Interior_Composition_Explorer
For some compact stars, a gravitational redshift has also been measured...
https://en.wikipedia.org/wiki/Gravitational_redshift#Astronomical_observations