Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 10/04/2018 20:58:55
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Over a million or billion year interval the photons coming from a star or galaxy have to miss every intervening particle or rock or comet. How do all those photons get through?
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A testament to the awesome emptiness of space.
There are parts where there is enough junk in the way to occlude the stars (the dark patches in our own milky way are due to large dust clouds which scatter the light).
https://en.wikipedia.org/wiki/Interstellar_medium
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How do all those photons get through?
Exoplanet hunters using the transit method (eg the Kepler space telescope) have shown that even when there is a good Jupiter-sized planet between us and a distant star, it only changes the brightness of the star by a few percent; this is undetectable by the human eye. Most of the photons miss the planet, leaving the total brightness of the star almost unchanged.
The Earth would dim the Sun's light by only 0.008%. A comet would have far less impact.
See: https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry
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Over a million or billion year interval the photons coming from a star or galaxy have to miss every intervening particle or rock or comet. How do all those photons get through?
As you'll recall, space is almost entirely void of all matter. However there is still interstellar dust and gas:
Interstellar Dust and Gas
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/dust.html
We often think of the vast areas of space between the stars as being completely empty. However, this is not really true. Much of the space between the stars is filled with atomic and molecular gas (primarily hydrogen and helium) and tiny pieces of solid particles or dust (composed mainly of carbon, silicon and oxygen). In some places this interstellar material is very dense, forming nebulas. In other regions the gas and dust density is very low. The image to the right shows an infrared view of the gas and dust in our galaxy along the plane of our Milky Way galaxy.
and
Interstellar Dust and Its Effect on Starlight
http://archive.seattleastro.org/webfoot/jul00/pg2.htm
INTERSTELLAR dust and gas are abundant in our galaxy and affect the way a star appears. In this article, we will look at the effect dust can have on stellar magnitude, distance estimates, and color. The average distribution of dust within our galaxy is on the order of one dust particle per million cubic meters. Although this sounds like an insignificant amount, interstellar dust makes up about 1% of the interstellar matter, and significantly reduces light received from distant sources.
There are other things in interstellar too which are the molecules he mentioned such as amino acids. Cool! :)
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Over a million or billion year interval the photons coming from a star or galaxy have to miss every intervening particle or rock or comet. How do all those photons get through?
Let's look at some numbers. The density of the local cloud of the interstellar medium has a density of ~0.1 particle per cc.
At sea level, air has a density of ~1.225kg/cc. Largely being made of nitrogen at ~2.34e-26 kg per atom, 1cc of sea level air contains ~5.24e19 atoms. So how long would a column of interstellar space with a 1 square cm cross-section need to be to contain as many particles as 1 cc of air? About 553.5 light years. If our atmosphere was a uniform density from top to bottom, it would be 8.5 km thick. his means that for our column of interstellar medium to contain the same number of particles as there is in a 1 square cross-section column of air with the same thickness of of atmosphere, it would have to be 470 million light years long. Light passes pretty well through the thickness of our atmosphere, and the individual stars we see are, at most, 1000's of light years away.
When it comes to viewing distant galaxies, we are looking in directions non-parallel to the disk of the galaxy and most of the space the light is passing through is intergalactic space with a density of ~0.000001 particle per cc. Now we are talking about distances that are large compared to the size of the observable universe before the light attenuation begins to even come close to being equal to that caused by passing through our atmosphere. In other words, the air surrounding us is much more a detriment to the observation of distant stars and galaxies than all the intervening matter in space is.
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In a word: lots of photons, not much stuff.
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At sea level, air has a density of ~1.225kg/cc
Any (ideal) gas has a volume of 24 liters per mole, at "room temperature" of 25C.
- Air is mostly N2 (28g/mole) with 20% O2 (32g/mole).
- Lets call it 29g/mol.
- The density of air is about 1.2 g/liter (not 1.2kg per cc).
- I think a factor of 1,000 got transferred from denominator to numerator?
See: https://en.wikipedia.org/wiki/Molar_volume#Ideal_gases
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At sea level, air has a density of ~1.225kg/cc
Any (ideal) gas has a volume of 24 liters per mole, at "room temperature" of 25C.
- Air is mostly N2 (28g/mole) with 20% O2 (32g/mole).
- Lets call it 29g/mol.
- The density of air is about 1.2 g/liter (not 1.2kg per cc).
- I think a factor of 1,000 got transferred from denominator to numerator?
See: https://en.wikipedia.org/wiki/Molar_volume#Ideal_gases
Sorry, typo. I forgot the e-6. It should read "At sea level, air has a density of ~1.225e-6 kg/cc." This is the actual value I used in the calculation.(1.225 kg/cc would give 5.19e25 atoms per cc not 5.19e19 atoms per cc)
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Would the stars look brighter if the Universe were not expanding?
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Apart from stars visible through a telescope in the andromeda galaxy the stars we see with the naked eye are in our own milky way. So we cannot associate expansion with them. The stars in more distant galaxies are not readily visible as individual entities so it is best to discuss expansion in terms of galaxies.
http://earthsky.org/tonight/can-we-see-stars-outside-our-milky-way-galaxy
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Would the stars look brighter if the Universe were not expanding?
Until Hubble in the 1930s, astronomers thought the universe was not expanding.
- The German astronomer Olbers (in 1823) challenged this view by suggesting that in a static universe, if you draw any line of sight, it should end up on a star, with a surface temperature of thousands of degrees.
- Therefore the whole sky should be a blazing inferno - which we don't see.
- This became known as Olbers' Paradox, even though other thinkers had posed the same question previously
- We now know that the redshift arising from the expansion of the universe can account for this apparent paradox
- So the stars look redder and dimmer than in a non-expanding universe
See: https://en.wikipedia.org/wiki/Olbers%27_paradox