APOD: Andromeda over the Sahara Desert (2022 Jul 11)

[APOD Robot]

Andromeda over the Sahara Desert

Explanation: What is the oldest thing you can see? At 2.5 million light years distant, the answer for the unaided eye is the Andromeda galaxy, because its photons are 2.5 million years old when they reach you. Most other apparent denizens of the night sky -- stars, clusters, and nebulae -- appear as they were only a few hundred to a few thousand years ago, as they lie well within our own Milky Way Galaxy. Given its distance, light from Andromeda is likely also the farthest object that you can see. Also known as M31, the Andromeda Galaxy dominates the center of the featured zoomed image, taken from the Sahara Desert in Morocco last month. The featured image is a combination of three background and one foreground exposure -- all taken with the same camera and from the same location and on the same calendar day -- with the foreground image taken during the evening blue hour. M110, a satellite galaxy of Andromenda is visible just above and to the left of M31's core. As cool as it may be to see this neighboring galaxy to our Milky Way with your own eyes, long duration camera exposures can pick up many faint and breathtaking details. Recent data indicates that our Milky Way Galaxy will collide and combine with the similarly-sized Andromeda galaxy in a few billion years.

<< Previous APOD

This Day in APOD

Next APOD >>

[VictorBorun]

is Triangulum Galaxy a thing you can see ?
Because its photons must be 3.2 million years old

I wonder if it is already possible to photograph the landscape illuminated by starlight alone.
That would illuminate dusty lanes in Andromeda's periphery as much as the dunes in the landscape

[XgeoX]

is Triangulum Galaxy a thing you can see ?
Because its photons must be 3.2 million years old

They start off with...
“ What is the oldest thing you can see?”
Then they qualify it with a “likely” later on in the post…
“ Given its distance, light from Andromeda is likely also the farthest object that you can see.”
“Oldest” I guess you could take to mean as to which galaxy has been around longest in which case you have to include the Milky Way as well as the Magellanic clouds etc…
But considering how awful light pollution is these days most likely Andromeda (with a 3.44 magnitude) is the most distant naked eye object people will see vs. M33 with a 5.72 visual magnitude.
Remarkably the most distant event ever that could have been seen was a gamma ray burst at a distance of 7.5 billion light years detected by NASA's Swift satellite in 2008. During it’s brief outburst it reached a naked eye level of 5 to 6! A record that is unlikely to be broken any time soon!

[De58te]

Actually Andromeda is NOT the oldest thing I can see with the unaided eye.Where I live in Canada I can drive a hundred miles north to the Canadian Shield and they say that the rock cuts they cut out by the side of the road are up to 4 BILLION years old! (The rocks, not the cuts.)

[heehaw]

Actually Andromeda is NOT the oldest thing I can see with the unaided eye.Where I live in Canada I can drive a hundred miles north to the Canadian Shield and they say that the rock cuts they cut out by the side of the road are up to 4 BILLION years old! (The rocks, not the cuts.)

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.

[beryllium732]

How big is Andromeda compared to the moon size seen from earth? I guess the long exposure picture isn't what the unaided eye will see from the same place at the same time?

Always thought that the unavoidable collision between our galaxies will turn out to a countorless elliptical galaxy in the end.

[VictorBorun]

Always thought that the unavoidable collision between our galaxies will turn out to a counterless elliptical galaxy in the end.

3 spins: Milky Way's, Andromeda's and the one of their orbiting their center of mass, — can not cancel themselves because they have roughly the same direction.
Milkdromeda will spin and end up a significantly compressed ellipsoid

[Chris Peterson]

Actually Andromeda is NOT the oldest thing I can see with the unaided eye.Where I live in Canada I can drive a hundred miles north to the Canadian Shield and they say that the rock cuts they cut out by the side of the road are up to 4 BILLION years old! (The rocks, not the cuts.)

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.

Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.

[Chris Peterson]

How big is Andromeda compared to the moon size seen from earth? I guess the long exposure picture isn't what the unaided eye will see from the same place at the same time?

Photographically it's about three times the size of the Moon. Visually it's perhaps half the size of the Moon.

[johnnydeep]

Actually Andromeda is NOT the oldest thing I can see with the unaided eye.Where I live in Canada I can drive a hundred miles north to the Canadian Shield and they say that the rock cuts they cut out by the side of the road are up to 4 BILLION years old! (The rocks, not the cuts.)

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.

Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.

Are the photons hitting the detectors in telescopes in space even the same ones that left their source in a far away star or galaxy?

[DL MARTIN]

Unless Andromeda has remained static for the last 2.5 million years, how do we account for the intervening variables in it's existence. For example, is this where the elusive dark energy and matter hang out?

[VictorBorun]

Actually Andromeda is NOT the oldest thing I can see with the unaided eye.Where I live in Canada I can drive a hundred miles north to the Canadian Shield and they say that the rock cuts they cut out by the side of the road are up to 4 BILLION years old! (The rocks, not the cuts.)

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.

Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.

How old is a photon? Ever young in its reference.
Is the photon one and the same as its wave propagates, reflects, scatter, refracts? It is.
Is the photon one and the same if it is split in two? It does not matter here.

What matters is this:
1) when you look at a rock here on Earth you can see it as it was a few microseconds ago
2) when you look at a dusty lane in Andromeda you can see it as it was 2.5 million years ago

[johnnydeep]

I noticed some extended and sharply bordered dark patches near the core of M110 in some images, notably this one:

Click to view full size image

Are they real objects - large dust clouds perhaps - or just random patterns?

[Chris Peterson]

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.

Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.

How old is a photon? Ever young in its reference.
Is the photon one and the same as its wave propagates, reflects, scatter, refracts? It is.
Is the photon one and the same if it is split in two? It does not matter here.

What matters is this:
1) when you look at a rock here on Earth you can see it as it was a few microseconds ago
2) when you look at a dusty lane in Andromeda you can see it as it was 2.5 million years ago

Or, if you look at it from the viewpoint of special relativity, you see everything as it is now.

[Chris Peterson]

Unless Andromeda has remained static for the last 2.5 million years, how do we account for the intervening variables in it's existence. For example, is this where the elusive dark energy and matter hang out?

It doesn't matter.

Dark energy does not "hang out" here. Most of the mass of Andromeda is dark matter, just as it is with the Milky Way and most galaxies.

[johnnydeep]

Unless Andromeda has remained static for the last 2.5 million years, how do we account for the intervening variables in it's existence. For example, is this where the elusive dark energy and matter hang out?

It doesn't matter.

Dark energy does not "hang out" here. Most of the mass of Andromeda is dark matter, just as it is with the Milky Way and most galaxies.

Dark matter "hangs out" everywhere though, right? That is, even near, around, and even IN the Earth and our bodies, dark matter suffuses otherwise empty space at a very low density. Or at least, so implies this recent XKCD comic - https://xkcd.com/2643/

Click to view full size image

And isn't the same true of dark energy, in that all space - even nearby space - has it as a property at least?

[ Arg, so many annoying questions: is dark matter compressible by gravity? Does it "clump up" in the presence of other matter? Are there dark matter black holes? Is dark matter always "in flux" - that is, travelling at some high speed - through space in the same way that photons are? Considering that we don't even know what dark matter IS, I assume some of these questions are impossible to answer. ]

[Chris Peterson]

Unless Andromeda has remained static for the last 2.5 million years, how do we account for the intervening variables in it's existence. For example, is this where the elusive dark energy and matter hang out?

It doesn't matter.

Dark energy does not "hang out" here. Most of the mass of Andromeda is dark matter, just as it is with the Milky Way and most galaxies.

Dark matter "hangs out" everywhere though, right? That is, even near, around, and even IN the Earth and our bodies, dark matter suffuses otherwise empty space at a very low density. Or at least, so implies this recent XKCD comic - https://xkcd.com/2643/

Click to view full size image

And isn't the same true of dark energy, in that all space - even nearby space - has it as a property at least?

[ Arg, so many annoying questions: is dark matter compressible by gravity? Does it "clump up" in the presence of other matter? Are there dark matter black holes? Is dark matter always "in flux" - that is, travelling at some high speed - through space in the same way that photons are? Considering that we don't even know what dark matter IS, I assume some of these questions are impossible to answer. ]

Presumably dark matter is everywhere, just like ordinary matter. But in both cases, we normally don't take much notice except where gravity causes concentrations. Dark energy appears to be a property of space itself, but only operates over cosmological distances.

[orin stepanek]

Is that M33 (>TriangulumGalaxy) hanging below the Andromeda
Galaxy?
Ti's a very beautiful photo taken from the Sahara! Kudos; Jordi Coy!

[Ann]

Andromeda over the Sahara Desert. Credit & Copyright: Jordi Coy

---

Is that M33 (>TriangulumGalaxy) hanging below the Andromeda
Galaxy?

M31 versus M33 (at bottom right). Image Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors)

---

No, M33 is not visible in today's APOD. It would be located quite some distance "to the lower right" in today's APOD.

Take a look at the APOD from September 26, 2013, to see the relative positions of Andromeda and M33 in the sky.

What you see apparently hanging "below" Andromeda is blue star Nu Andromedae and what might be a pair of cool faint companions.

Nu Andromedae and companions. R/B-band color composite image from the Second Digitized Sky Survey (DSS2), measuring 30 arcminutes across.

---

Ann

[orin stepanek]

Andromeda over the Sahara Desert. Credit & Copyright: Jordi Coy

---

Is that M33 (>TriangulumGalaxy) hanging below the Andromeda
Galaxy?

M31 versus M33 (at bottom right). Image Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors)

---

No, M33 is not visible in today's APOD. It would be located quite some distance "to the lower right" in today's APOD.

Take a look at the APOD from September 26, 2013, to see the relative positions of Andromeda and M33 in the sky.

What you see apparently hanging "below" Andromeda is blue star Nu Andromedae and what might be a pair of cool faint companions.

Nu Andromedae and companions. R/B-band color composite image from the Second Digitized Sky Survey (DSS2), measuring 30 arcminutes across.

---

Ann

OK! Thanks Ann; I thought it was a little too close. I knew that M33 hung below Andromeda!

[johnnydeep]

It doesn't matter.

Dark energy does not "hang out" here. Most of the mass of Andromeda is dark matter, just as it is with the Milky Way and most galaxies.

Dark matter "hangs out" everywhere though, right? That is, even near, around, and even IN the Earth and our bodies, dark matter suffuses otherwise empty space at a very low density. Or at least, so implies this recent XKCD comic - https://xkcd.com/2643/

Click to view full size image

And isn't the same true of dark energy, in that all space - even nearby space - has it as a property at least?

[ Arg, so many annoying questions: is dark matter compressible by gravity? Does it "clump up" in the presence of other matter? Are there dark matter black holes? Is dark matter always "in flux" - that is, travelling at some high speed - through space in the same way that photons are? Considering that we don't even know what dark matter IS, I assume some of these questions are impossible to answer. ]

Presumably dark matter is everywhere, just like ordinary matter. But in both cases, we normally don't take much notice except where gravity causes concentrations. Dark energy appears to be a property of space itself, but only operates over cosmological distances.

I hope some day at least, we will be able to notice dark matter (and perhaps even dark energy) enough to be able to finally detect it here on Earth in a supercollider experiment or some such!

[Ann]

I noticed some extended and sharply bordered dark patches near the core of M110 in some images, notably this one:

Click to view full size image

Are they real objects - large dust clouds perhaps - or just random patterns?

These dark patches are real. They are the remnants of a dust cloud that gave rise to some stars some, oh, 100 million years ago? Like the Pleiades? Or longer ago, say, 300 million years ago, like the five middle stars of the Big Dipper? Because I think that the bluish stars near the center of NGC 205 are spectral class A.

Ann

[DL MARTIN]

Thanks to all for clarifying my "hanging out" remark on dark matter and energy. I'm not an astronomy whiz so I appreciate the feedback. What, I guess I find questionable is we don't seem to acknowledge the total lack of scrutiny between the time frame that the observed entity represents and the present time that we are observing it. For example and to reiterate, how does astronomy account for the intervening 2.5 million years of existence of Andromeda? Or, for that matter, the 8 minutes applied to the Sun.
It seems that there is a fundamental gap in knowledge between the 'ago' and the 'now' that is not explained.

[johnnydeep]

I noticed some extended and sharply bordered dark patches near the core of M110 in some images, notably this one:

Click to view full size image

Are they real objects - large dust clouds perhaps - or just random patterns?

These dark patches are real. They are the remnants of a dust cloud that gave rise to some stars some, oh, 100 million years ago? Like the Pleiades? Or longer ago, say, 300 million years ago, like the five middle stars of the Big Dipper? Because I think that the bluish stars near the center of NGC 205 are spectral class A.

Ann

Yeah, I guess they're real alright. This Hubble shot show one of them much more clearly and well defined, plus some smaller dark clouds closer to the nucleus that weren't visible in my pic:

Click to view full size image

[Ann]

Thanks to all for clarifying my "hanging out" remark on dark matter and energy. I'm not an astronomy whiz so I appreciate the feedback. What, I guess I find questionable is we don't seem to acknowledge the total lack of scrutiny between the time frame that the observed entity represents and the present time that we are observing it. For example and to reiterate, how does astronomy account for the intervening 2.5 million years of existence of Andromeda? Or, for that matter, the 8 minutes applied to the Sun.
It seems that there is a fundamental gap in knowledge between the 'ago' and the 'now' that is not explained.

What you are really complaining about is the fact that the speed of light is so slow, only 299792458 metres per second (approximately 300000 km/s or 186000 mi/s).

That's why it takes 2.5 million years for light emitted by the Andromeda Galaxy to travel all the way to us. (And that's why we don't know the present state of Betelgeuse, of Sgr A*, the black hole at the center of our galaxy, or of Voyager I and II, or even of the Sun! And that's why there will be a fourteenth of a second of delay when you speak on the phone with someone on the other side of the Earth!)

Isn't that a bother? Wouldn't it be better if the speed of light was infinite? Then we would really know of any major developments going on in Andromeda right now. We would know right away if Sgr A* was up to anything unusual. We could listen to the news on the radio about the current developments in the Virgo Cluster regarding supernovas and outbursts and such things, and the news would really be current! Wouldn't that be a good thing?

Maybe. Or not. I found this question and an answer at Quora:

What would the universe be like if the speed of light was infinite?

Adam Wu, Amateur scientist answered:

The speed of light is actually the speed of causality.

A universe where the speed of causality is infinite is a universe with no time. Every effect happens instantaneous with its cause. Both the beginning and the end of the universe happen simultaneously, so the universe would never exist, and nor could anything else.

Oh wow. Well, that would not be so good, would it? Not all things considered.

I guess we will just have to take our Universe as it is. We may mutter and complain that we don't know what Andromeda is like now, but maybe we can find some consolation in the realization that our ignorance regarding Andromeda (and everything else that is some distance away from us - what did your auntie say on the phone during that one fourteenth of a second of delay during your conversation with her?) is the price for our existence.

To never know and yet to live, or know at once and die right now, I guess that is the question!

---

Ann