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### Course: Cosmology and astronomy>Unit 2

Lesson 1: Life and death of stars

# Star field and nebula images

Star Field and Nebula Images. Created by Sal Khan.

## Want to join the conversation?

• If there are an infinite number of stars and galaxies in the universe, why is there still so much empty, nonluminous space in the background of these images?
• If the universe is infinite and expanding, then there is a point where parts of the universe that are distant enough are moving away from us at faster than the speed of light, which means the light of bodies in those parts will never reach us. What we'd see, if we were able to actually see that far, is the light of objects would red shift further and further until the light pretty much 'flat-lined', at which point we couldn't see anymore objects. That leaves us with a finite visible volume for which there is finite matter and finite stars and lots of room for empty space.

There are problems with this as we don't know if our universe is infinite, and the age of the universe doesn't allow us to currently see any further than about 13.7B ly. Keep in mind that, as you look further away, you're also looking back in time. So the current visible limit is shortly after the big bang occurred. So, in our observable universe, there are a finite number of stars and lots of empty space between.
• This is not related to the video, but what is kelvin? ( Yes, I know it's a measure of temperature, but how much is it in Celsius?)
(1 vote)
• Zero Kelvin is -273.15 degrees Celsius.
100 Kelvin is -173.15 degrees Celsius
273.15 Kelvin is 0 degrees Celsius
Just for kicks and giggles:
255.372 Kelvin is 0 degrees Fahrenheit
• Why are planetary nebulae named so? Do they form planets? Wouldn't planets be formed at an earlier stage of the star than the Red Giant phase?
• As per Wikipedia: The term "planetary nebula" is a misnomer because they are unrelated to planets. The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet".
• - If a photon from a star took 7000 light years to reach us, wouldn't the star be greater than 7000 light years away from us due to the expansion of space?
• Expansion isn't strong enough to overcome the attraction of gravity at distances within the local galactic neighborhood.
• How does a nebula form?
• (Here is a basic answer, I don't know too much! :D)
Nebulae can form from the gas and dust from a supernova. Sal said Planetary nebulae form from the shedding of the outer layers of a star in the middle of it.
I hope that helped!
(1 vote)
• if light is the fastest thing in the universe, how could you detect the supernova itself which blew near the eagle nebula but not detect its effects? shouldnt the light energy coming from the supernove reach at the same time or a little before the light showing the effects of the supernova?
• I am not sure what the difference in "light energy coming from the supernove" and "light showing the effects of the supernova".

Are you asking about how we can detect that a supernova happened before we see the star brighten? When a star is in the process of going supernova the reaction starts in the core of the star and takes time to propagate out to the surface but there are a large number of neutrinos created in the reaction which only rarely interact with other particles so they can travel out from the core of the star without being interfered with while the supernova reaction is still propagating to the surface of the star. So we get a burst of neutrinos before we get the light from the supernova.
• Why is a lightyear called a lightyear when its a distance and not time?
• Because it is the distance light travels in a year. It is just like a composite unit. The speed of light is in units of distance/time and year is time so (distance/time)*time so the times cancel out leaving you with distance.
• If we are seeing the Eagle Nebula 7000 years ago because its 7000 light years away, would we see Earth 100 years ago if we traveled 100 light years away from Earth?