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Looking at pictures like this

enter image description here

I bet it is a picture of a giant cloud of dust, bigger than a galaxy and made of asteroids and planets (instead of stars).

But probably I'm wrong. What is that?

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It's much smaller than the galaxy. What it's made of is a more complicated question that I'll leave to the answers, but I just want to note that this is a certain neighborhood in our galaxy of dust and stars, and much much smaller than the Milky Way itself. – Alan Rominger May 18 '11 at 16:30
up vote 4 down vote accepted

It's the Eagle Nebula as pictured by the Hubble Space Telescope, also known as the "Pillars of Creation" picture.

The Eagle Nebula is a young cluster of stars in the Constellation Serpens that began to clump just 5.5 million years ago:

The distance is 7,000 light years, so it's in the Milky Way (whose diameter is about 100,000 light years), of course. So yes, there is gas and dust but it's going to collapse into stars at some point.

The size of the nebula is just 70 x 55 light years or so. The linear dimensions are about 10 times larger than the distance between the Sun and another closest star so such a volume of space could normally be expected to contain $O(1,000)$ stars or so.

If you wanted the same apparent object to have distance e.g. 7 million instead of 7 thousand light years, which would be approximately needed for your "galactic" interpretation, the cloud would have to contain a million times more matter or so - to send the right number of photons here. Such an object would be comparably heavy as a galaxy but it would also move away from us and produce a detectable redshift.

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But what illuminates it? It has internal stars, or it just reflects and blocks external stars' light? – Jader Dias May 18 '11 at 17:00
Dear @Jader, it's shining itself. The temperatures of Nebulae go from 1 kelvin to 1 million kelvins. In the Eagle Nebula above, the bluish fog around the picture is, for example, at 170 kelvins. So it has enough energy to excite some atoms and shine. As the clump keeps on shrinking, the energy needed to reheat the gas is coming from the gravitational energy. Of course, when stars start to burn, there's another source of energy and light. – Luboš Motl May 18 '11 at 17:23
@Jader Also keep in mind that the Hubble pictures aren't true-color. The telescope uses different filters to isolate certain molecular spectral lines, and then maps those out to colors that we humans can see. – voithos May 18 '11 at 18:01
To clarify, these are starforming clouds and are pretty cold, probably in the neihghborhhod of 10K, you are not seeing thermally radiated light. We have reflection nebula, which shine by reflected (or often just slightly diffracted) light from nearby starts. We also have emission nebula, which are a lot like florescent materials, converting UV radiation into light. The UV comes from young massive stars (usually birthed in the same nebula) whose thermal output is largely in the UV. – Omega Centauri May 18 '11 at 20:04

The above image was taken in optical wavelengths, in the inra red large portions are transparent, see :, which describes the same object as viewed by NASA's Spitzer space telescope.

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