APOD: Veil Nebula: Wisps of an Exploded Star (2017 Sep 19)

[sallyseaver]

It's not that we don't care...
it's simply that we don't know precisely when the explosion "actually" happened.

Hence, we'll just call the accurately known spacetime interval Δs "the time":

In four-dimensional spacetime, the analog to distance is the spacetime interval Δs:

---

The spacetime interval Δs between any two events is independent
of the inertial frame of reference in which they are recorded.

I'd like to test for understanding.

I get the space-time interval (from past physics coursework), and understand where you are coming from. However, we now know that the distance between galaxy clusters and galaxies is expanding at an accelerated rate.

So even though Hubble's constant is about 67.8 km/s per mega parsec (i.e. only discernible at mega-parsec distances and it is a constant), that rate of expansion was only valid for a certain moment in time. 7000 light years is too small a distance for it to be relevant for us observing the evolution of the Veil Nebula, but shouldn't the accelerated expansion of space matter when think about the distance in the past that we are observing?

For example, NGC 1300 is measured as being 70 x 10^6 ly (or about 21.5 mega-parsecs) away. We picture some time T_o in the past when Earth and NGC 1300 were a given distance D_o away from each other, then space expanded between both objects. The expansion moves Earth a distance Dist(t) from the location at T_o AND it moves NGC 1300 the same amount from the location it had at T_o. So the current distance between Earth and NGC 1300 is D_o + 2 Dist(t). [using spherical coordinates, the motion involves one radial direction] "Dist(t)" means distance as a function of time, and we know this function includes an acceleration term.

Let's say that D_o was 10 x 10^6 ly. Then by current time t_c, Earth and NGC 1300 have moved apart by 60 x 10^6 ly. Thus: D_o + 2 Dist(t_c) = 70 x 10^6 ly = 10 + (2x30) mega light years. In this case, the light from NGC 1300 seen on earth at t_c did not start its journey 70 x 10^6 ly in the past, rather (10 + 30) = 40 x 10^6 ly.

Do you see a problem with this reasoning?

[MarkBour]

...Yes, it would seem that Cygnus X-5 is the X-ray source of the Cygnus Loop with the Veil Nebula.
... But nothing seems to be known about Cygnus X-5. Wikipedia does not provide a link to more information about Cygnus X-5. So we can't say that Cygnus X-5 is a black hole.
Ann

Thanks! Okay, we'll have to ask Timothy Leary. Or maybe someone will investigate it.

[MarkBour]

---

I'd like to test for understanding. ...

I will be interested to see neufer's answer to this as well.

[bjmb]

splendid picture, but somewhat sloppy caption> 'Wisps like this are all that remain visible of a Milky Way star. About 7,000 years ago that star exploded in a supernova leaving the Veil Nebula. At the time, the expanding cloud was likely as bright as a crescent Moon, remaining visible for weeks to people living at the dawn of recorded history.' scientifically exact would be: 'Wisps like this are all that remain visible of a Milky Way star. About 7,000 years ago that star's explosion in a supernova became visible on earth, leaving the Veil Nebula. At the time, the expanding cloud was likely as bright as a crescent Moon, remaining visible for weeks to people living at the dawn of recorded history.' the explosion itself was of course dusted and done 7,000 years ago

No, the way it's worded is fine. We don't care when the explosion "actually" happened. What is relevant is when the light reached Earth. This is almost always true, and almost always the way things are dated astronomically. It is normal to treat the date of occurrence as the date of observation.

as the topic is still being discussed, i'll venture forth once more. 'we don't care' is in no astromer's handbook, and if 'we' is astronomers, please note that the captions are written for a general public. treating the date of observation as the date of occurrence was fine in tycho brahe's time, but we've advanced a lot and we know that the date of observation is not the date of occurrence. the fact that we don't know the date of occurrence is no reason to just fake a date of occurrence. and lastly, what is so dastardly difficult or degrading or unscientific about saying 'about 7.000 years ago the explosion could be seen in the sky'?

[Chris Peterson]

No, the way it's worded is fine. We don't care when the explosion "actually" happened. What is relevant is when the light reached Earth. This is almost always true, and almost always the way things are dated astronomically. It is normal to treat the date of occurrence as the date of observation.

as the topic is still being discussed, i'll venture forth once more. 'we don't care' is in no astromer's handbook, and if 'we' is astronomers, please note that the captions are written for a general public. treating the date of observation as the date of occurrence was fine in tycho brahe's time, but we've advanced a lot and we know that the date of observation is not the date of occurrence. the fact that we don't know the date of occurrence is no reason to just fake a date of occurrence. and lastly, what is so dastardly difficult or degrading or unscientific about saying 'about 7.000 years ago the explosion could be seen in the sky'?

Well, in fact, we astronomers genuinely don't care when something like this happened. We don't need to say it because we understand it to be so. Read any paper on a supernova, there will be no reference to "real" timing. Timing is tied to observation. There may be a reference to the approximate distance to the object, but if so, it's just to make clear details of the observational technique, not to establish timing.

When we astronomers do talk about it, it's normally in response to discussions like this, where we point out to people who aren't astronomers why the idea of a "real time" doesn't even make much sense physically.

[Hans Sander]

In my opinion one of the best pictures recently.

[BDanielMayfield]

It's not that we don't care...
it's simply that we don't know precisely when the explosion "actually" happened.

Hence, we'll just call the accurately known spacetime interval Δs "the time":

In four-dimensional spacetime, the analog to distance is the spacetime interval Δs:

---

The spacetime interval Δs between any two events is independent
of the inertial frame of reference in which they are recorded.

I'd like to test for understanding.

I get the space-time interval (from past physics coursework), and understand where you are coming from. However, we now know that the distance between galaxy clusters and galaxies is expanding at an accelerated rate.

So even though Hubble's constant is about 67.8 km/s per mega parsec (i.e. only discernible at mega-parsec distances and it is a constant), that rate of expansion was only valid for a certain moment in time. 7000 light years is too small a distance for it to be relevant for us observing the evolution of the Veil Nebula, but shouldn't the accelerated expansion of space matter when think about the distance in the past that we are observing?

For example, NGC 1300 is measured as being 70 x 10^6 ly (or about 21.5 mega-parsecs) away. We picture some time T_o in the past when Earth and NGC 1300 were a given distance D_o away from each other, then space expanded between both objects. The expansion moves Earth a distance Dist(t) from the location at T_o AND it moves NGC 1300 the same amount from the location it had at T_o. So the current distance between Earth and NGC 1300 is D_o + 2 Dist(t). [using spherical coordinates, the motion involves one radial direction] "Dist(t)" means distance as a function of time, and we know this function includes an acceleration term.

Let's say that D_o was 10 x 10^6 ly. Then by current time t_c, Earth and NGC 1300 have moved apart by 60 x 10^6 ly. Thus: D_o + 2 Dist(t_c) = 70 x 10^6 ly = 10 + (2x30) mega light years. In this case, the light from NGC 1300 seen on earth at t_c did not start its journey 70 x 10^6 ly in the past, rather (10 + 30) = 40 x 10^6 ly.

Do you see a problem with this reasoning?

I also wish to test my understanding Sally, so I will offer up my thinking on your question with the hope that if I'm wrong I too will receive a better answer.

Since the expansion of spacetime is universal, there is no need to multiply the change in distance by 2. Other than that I think your math is fine.

Bruce

[Chris Peterson]

7000 light years is too small a distance for it to be relevant for us observing the evolution of the Veil Nebula, but shouldn't the accelerated expansion of space matter when think about the distance in the past that we are observing?

Cosmological expansion isn't uniform. At short distances, gravity is much stronger, and it holds the fabric of spacetime together. The expansion between Earth and the Veil Nebula isn't just small... it's zero. Our galaxy isn't changing size due to the expansion of the Universe. The Earth isn't getting farther from the Sun. The distance between the Milky Way and Andromeda isn't changing (other than by ordinary Newtonian orbital dynamics).

Distance at cosmological scales is a complicated concept because of universal expansion, which is why there are different ways of defining it (yielding different values). Two of the most common are light travel time (units of time, but treated as a distance) and comoving distance (which considers how far apart two objects are "now" given the expansion that has occurred since the light was emitted. And there are a few others.

For a cosmologically nearby object like NGC 1300, all of these distances will have the same value within a fraction of a percent, and the variation in rate of expansion with time (the changing Hubble "constant") makes no significant difference. None of these things becomes significant until you have much greater redshift values (e.g. z=0.1, compared with z=0.005 for NGC 1300).

[sallyseaver]

It's not that we don't care...
it's simply that we don't know precisely when the explosion "actually" happened.

Hence, we'll just call the accurately known spacetime interval Δs "the time":

I'd like to test for understanding.

I get the space-time interval (from past physics coursework), and understand where you are coming from. However, we now know that the distance between galaxy clusters and galaxies is expanding at an accelerated rate.

So even though Hubble's constant is about 67.8 km/s per mega parsec (i.e. only discernible at mega-parsec distances and it is a constant), that rate of expansion was only valid for a certain moment in time. 7000 light years is too small a distance for it to be relevant for us observing the evolution of the Veil Nebula, but shouldn't the accelerated expansion of space matter when think about the distance in the past that we are observing?

For example, NGC 1300 is measured as being 70 x 10^6 ly (or about 21.5 mega-parsecs) away. We picture some time T_o in the past when Earth and NGC 1300 were a given distance D_o away from each other, then space expanded between both objects. The expansion moves Earth a distance Dist(t) from the location at T_o AND it moves NGC 1300 the same amount from the location it had at T_o. So the current distance between Earth and NGC 1300 is D_o + 2 Dist(t). [using spherical coordinates, the motion involves one radial direction] "Dist(t)" means distance as a function of time, and we know this function includes an acceleration term.

Let's say that D_o was 10 x 10^6 ly. Then by current time t_c, Earth and NGC 1300 have moved apart by 60 x 10^6 ly. Thus: D_o + 2 Dist(t_c) = 70 x 10^6 ly = 10 + (2x30) mega light years. In this case, the light from NGC 1300 seen on earth at t_c did not start its journey 70 x 10^6 ly in the past, rather (10 + 30) = 40 x 10^6 ly.

Do you see a problem with this reasoning?

I also wish to test my understanding Sally, so I will offer up my thinking on your question with the hope that if I'm wrong I too will receive a better answer.

Since the expansion of spacetime is universal, there is no need to multiply the change in distance by 2. Other than that I think your math is fine.

Bruce

Bruce,
Thank you for taking an interest in my post and checking my math. (I appreciate it.)

Is it possible in your thinking, that you are using the planet Earth as the center of the universe? As I understand it, the expansion does not prefer a particular direction with respect to Earth. An observer in any system (even a system with red-shift value 8 relative to Earth) would observe the same expansion. And from relativity, there is no preferred origin. From my own thinking, it seems to me that space would not just expand away from earth, but wherever a chunk of it is, it is pushing outward in all directions.

Please look at the following figure and see if you still believe that there is not a factor of 2 with space expansion away from a common original distance.



In the example I gave, t is the time it takes for Earth to move Dist(t) from its original position 30 x 10^6 ly. And 70 x 10^6 ly is the total distance between Earth and NGC 1300 now.

[sallyseaver]

7000 light years is too small a distance for it to be relevant for us observing the evolution of the Veil Nebula, but shouldn't the accelerated expansion of space matter when think about the distance in the past that we are observing?

Cosmological expansion isn't uniform. At short distances, gravity is much stronger, and it holds the fabric of spacetime together. The expansion between Earth and the Veil Nebula isn't just small... it's zero. Our galaxy isn't changing size due to the expansion of the Universe. The Earth isn't getting farther from the Sun. The distance between the Milky Way and Andromeda isn't changing (other than by ordinary Newtonian orbital dynamics).

Distance at cosmological scales is a complicated concept because of universal expansion, which is why there are different ways of defining it (yielding different values). Two of the most common are light travel time (units of time, but treated as a distance) and comoving distance (which considers how far apart two objects are "now" given the expansion that has occurred since the light was emitted. And there are a few others.

For a cosmologically nearby object like NGC 1300, all of these distances will have the same value within a fraction of a percent, and the variation in rate of expansion with time (the changing Hubble "constant") makes no significant difference. None of these things becomes significant until you have much greater redshift values (e.g. z=0.1, compared with z=0.005 for NGC 1300).

Chris,

You have pointed out the relative size of the expansion, that it would NOT be on the order of 30 x 10^6 ly with respect to NGC 1300. This is apparent in looking at an example. With constant Hubble expansion (67.8 km/sec per mega parsec), in 10,000 years (3.154 x 10^11 sec), gives 2.138 x 10^13 km which is only about 2.3 light years which is a negligible percent of 1 mega parsec.

[My understanding is that with the accelerated expansion, at high redshift values, that the expansion is approaching the speed of light. Is this your understanding?]

What is more important to me is whether there is a factor of 2 that could impact the time that light has been traveling from a very distant (redshift > 1.5) object. For example, galaxy GN-z11 has redshift 11.1, and it is said that we are seeing light from 13.4 billion years ago. However, if my formula is correct [current distance = D₀ + 2 Dist(t)] then one would need to know the initial distance between GN-z11 and the center of the Milky Way, when our galaxy first appeared [D₀]; and then divide the remaining distance in half and calculate -- with accelerated expansion -- the time it would take for space to expand this halved distance [Dist(t)]. Only then would we know the time involved for when the light we see now left GN-z11. What say you?

[BDanielMayfield]

Let's say that D_o was 10 x 10^6 ly. Then by current time t_c, Earth and NGC 1300 have moved apart by 60 x 10^6 ly. Thus: D_o + 2 Dist(t_c) = 70 x 10^6 ly = 10 + (2x30) mega light years. In this case, the light from NGC 1300 seen on earth at t_c did not start its journey 70 x 10^6 ly in the past, rather (10 + 30) = 40 x 10^6 ly.

Do you see a problem with this reasoning?

I also wish to test my understanding Sally, so I will offer up my thinking on your question with the hope that if I'm wrong I too will receive a better answer.

Since the expansion of spacetime is universal, there is no need to multiply the change in distance by 2. Other than that I think your math is fine.

Bruce

Bruce,
Thank you for taking an interest in my post and checking my math. (I appreciate it.)

Is it possible in your thinking, that you are using the planet Earth as the center of the universe? As I understand it, the expansion does not prefer a particular direction with respect to Earth. An observer in any system (even a system with red-shift value 8 relative to Earth) would observe the same expansion. And from relativity, there is no preferred origin. From my own thinking, it seems to me that space would not just expand away from earth, but wherever a chunk of it is, it is pushing outward in all directions.

Please look at the following figure and see if you still believe that there is not a factor of 2 with space expansion away from a common original distance.



In the example I gave, t is the time it takes for Earth to move Dist(t) from its original position 30 x 10^6 ly. And 70 x 10^6 ly is the total distance between Earth and NGC 1300 now.

It's very true that there is no preferred location, and that space expands in all directions at cosmological distances beyond gravitationally bound systems. However, I still believe that you have doubled the rate of expansion.

Perhaps I'm wrong, but it seems to me that if your math was correct it would have been simpler to have just doubled the Hubble "constant" in the first place.

Bruce