Star visible for longest time from Earth

[Nathi ORea]

TL;DR Summary

What would be a star that has been visible for many millions of years from Earth

I was just looking up at the stars wondering how long they have been visible for. I started wondering if any of them might have been around this galactic neighbourhood long enough and long lived/old enough that the dinosaurs could have seen it. Do you think they’d be any?

If not what might be a star which has probably been visible from Earth for perhaps 10’s of millions years and is likely to be for a long time still?

I was thinking maybe a star like Canopus might be as it is reasonably long-lived, but still has a high absolute magnitude. I remember reading at one stage that it has been the brightest star in the sky before Sirius took that podium, and will take over again when Sirius starts drifting away.

I’d be really interested in any insights.

 

[Astronuc]

Summary:: What would be a star that has been visible for many millions of years from Earth

I was just looking up at the stars wondering how long they have been visible for. I started wondering if any of them might have been around this galactic neighbourhood long enough and long lived/old enough that the dinosaurs could have seen it. Do you think they’d be any?

If not what might be a star which has probably been visible from Earth for perhaps 10’s of millions years and is likely to be for a long time still?

I was thinking maybe a star like Canopus might be as it is reasonably long-lived, but still has a high absolute magnitude. I remember reading at one stage that it has been the brightest star in the sky before Sirius took that podium, and will take over again when Sirius starts drifting away.

I’d be really interested in any insights.

Besides the sun?

Or all the stars in the Milky Way (our) galaxy? And the Andromeda galaxy? And the Magellanic clouds?

https://spaceplace.nasa.gov/galaxies-age/en/

Most galaxies are between 10 billion and 13.6 billion years old. Our universe is about 13.8 billion years old, so most galaxies formed when the universe was quite young!
Astronomers believe that our own Milky Way galaxy is approximately 13.6 billion years old. The newest galaxy we know of formed only about 500 million years ago.

But a revised estimate indicates the Milky Way galaxy is ~10 billion years old, or about the same age as the Andromeda galaxy.
https://www.sciencealert.com/starquake-data-reveals-a-new-way-to-measure-the-age-of-the-milky-way
https://lowell.edu/views-from-mars-hill-andromeda-galaxy/

In contrast, the Earth is thought to be 4.5 billion years old.
https://Earth'sky.org/earth/age-of-earth-how-old-is-planet-earth/
https://www.scientificamerican.com/article/how-science-figured-out-the-age-of-the-earth/#

two stars that make up Alpha Centauri, Rigil Kentaurus and Toliman, are quite similar to our sun. Rigil Kentaurus, also known as Alpha Centauri A, is a yellowish star, slightly more massive than the sun and about 1.5 times brighter. Toliman, or Alpha Centauri B, has an orangish hue; it’s a bit less massive and half as bright as the sun. Studies of their mass and spectroscopic features indicate that both these stars are about 5 to 6 billion years old, slightly older than our sun.

https://Earth'sky.org/brightest-stars/alpha-centauri-is-the-nearest-bright-star/

Edit/update: I should point out that the relative position of the stars we see today from Earth would look very different millions or billions of years ago, because stars move in the galaxy, and galaxies move with respect to each other.

 

[Vanadium 50]

When I read the subject line, I had a completely different interpretation - one where the answer was "Polaris".

@Astronuc has an excellent answer. The problem with galactic stars is that the galaxy has rotated over a quarter turn since the age of dinosaurs. Long lived stars tend to be dim, so they need to be relatively close, but now it becomes unusual to stay relatively close.

Let's take Canopus (ignoring another problem discussed below) as an example. Say it's 300 ly away. It's proper motion is about 30 mas/year or 14 km/s, and its radial motion is 20 km/s, so it's moving relative to the sun at 25 km/s. That means its moved about 5500 ly in that time. While not quite "on the other side of the galaxy", it's not exactly close. It would be just barely visible.

However, Canopus is only 25M years old. Here's your real problem - because of rotation, you need stars that are a) visible from great distances, and because of these distances they need to be bright, and bright stars don't last very long.

 

[Astronuc]

Long lived stars tend to be dim, so they need to be relatively close, but now it becomes unusual to stay relatively close.

Yeah, I didn't address young versus old stars.

For example, the Pleiades stars
https://en.wikipedia.org/wiki/Pleiades#Age_and_future_evolution

Ages for star clusters can be estimated by comparing the Hertzsprung–Russell diagram for the cluster with theoretical models of stellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated.

B. Zuckerman and Inseok Song, "Young Stars near the Sun," Annu. Rev. Astron. Astrophys. 2004. 42:685–721 doi: 10.1146/annurev.astro.42.053102.134111
http://www.ifa.hawaii.edu/~reipurth/reviews/zuckerman_araa.pdf

Until the late 1990s the rich Hyades and the sparse UMa clusters were the only coeval, comoving concentrations of stars known within 60 pc of Earth. Both are hundreds of millions of years old. Then beginning in the late 1990s the TW Hydrae Association, the Tucana/Horologium Association, the β Pictoris Moving Group, and the AB Doradus Moving Group were identified within∼60 pc of Earth, and the η Chamaeleontis cluster was found at 97 pc. These young groups (ages 8–50 Myr), along with other nearby, young stars, will enable imaging and spectroscopic studies of the origin and early evolution of planetary systems.

NGC 2547 star cluster - "Although their exact ages remain uncertain, astronomers estimate that NGC 2547’s stars range from 20 to 35 million years old," officials from the ESO wrote in a statement.
https://www.space.com/20620-blue-star-cluster-photo.html

The estimated age of the system is between 200 and 300 million years. The Sirius system originally consisted of two bright blue stars. Alpha Canis Majoris B was the more massive one, with roughly 5 solar masses, but then it consumed its resources and evolved into a red giant before expelling its outer shell and becoming a white dwarf some 120 million years ago.

https://www.constellation-guide.com/sirius-the-dog-star/

Including the pre-WD evolutionary timescale of the assumed progenitor, the total age of Sirius B is about 228 ± 10 Myr.

The age of Sirius A based on these models is about 237-247 Myr, with uncertainties of ±15 Myr, consistent with that of the WD companion.

https://ui.adsabs.harvard.edu/abs/2017ApJ...840...70B/abstract
https://en.wikipedia.org/wiki/Sirius

Antares is a relatively young star

Comparison with theoretical evolutionary tracks suggests a mass of 15 ± 5 M⊙ with an age of 11-15 Myr, which is consistent with the recently estimated age for the Upper Scorpius OB association.

https://ui.adsabs.harvard.edu/abs/2013A&A...555A..24O/abstract
https://www.aanda.org/articles/aa/full_html/2013/07/aa21063-13/aa21063-13.html
https://en.wikipedia.org/wiki/Antares

And stars like the sun are ~4.5 +/- ~0.5 billion years.

Aldebaran, a red giant, has an age ~6.4 +1.4/-1.1 billion years
https://en.wikipedia.org/wiki/Aldebaran
http://pure-oai.bham.ac.uk/ws/files/54771082/Farr_2018_ApJL_865_L20.pdf

 

[snorkack]

I tried to look at it with this logic:
Assume a visibility cutoff at magnitude +6.
Now, as for peculiar velocities, Alpha Centauri´s 32 km/s is quoted as about the peak of velocity distribution for nearby stars.
A star with absolute magnitude 0 (about 90 times brighter than Sun) would fade to magnitude +6 at about 160 pc from Sun. If it passes near Sun, it is visible over an arc of 320 pc.
An approximation is that 1 km/s for 1 million years is 1 pc. So a star with absolute magnitude 0 would be visible for 10 million years. Its main sequence lifetime would be much longer, over 100 million years.
A star with absolute magnitude -2 would be visible for 25 million years at the same speed. So the breakpoint, where brighter stars tend to burn out before they move out of sight, would be somewhere brighter than absolute magnitude -2.
That 32 km/s, however, is a distribution. For an individual star visible for longest time, you should be looking for stars somewhat dimmer than the breakpoint, but also having unusually small peculiar velocity.

 

[sophiecentaur]

However, Canopus is only 25M years old. Here's your real problem - because of rotation, you need stars that are a) visible from great distances, and because of these distances they need to be bright, and bright stars don't last very long.

This all makes me wonder what the OP is actually after with his / her question. There are a lot of variables involved and, without including all of them, you can't make a reasonable prediction. There may have been stars around that were very bright before humans were around and which have faded to White Dwarfs long ago. We wouldn't necessarily know about them.

There are a few predictions that 'make sense' in terms of future observations - like the Earth's precession / slow wobble that will mean Polaris's polar position will be taken by other stars over the next few thousand years. Gamma Cephi will take its position as (nearest to) Pole Star in around 4000 years. Now that's a time scale that we could hope for future humans to be aware.

 

[Vanadium 50]

What you are kind of looking for is a star of around 7 solar masses. 65M years ago it was a blue main sequence star like Achernar (α Eri) visible from thousand of light years away, and today it's a red giant star also visible from thousands of light years away. It is probably too dim to have a name, just a designation.