What causes the 'starburst' galaxies to have the distinct red hue?

Question

I have a general understanding of why they enter the starburst phase (cannibalization, etc.) and what they are (active star forming regions), but what I'm curious about is why they all seem to share the red glow characteristic. M51 is a great example of this. I'm pretty sure it's not a false colour or a multi-wavelength overlay.

It's just something I don't quite understand. Hopefully someone can shine some light on this for me.

Answer 1

To say that starburst galaxies are "red" is an oversimplification. Generally a "red" galaxy is a galaxy containing little gas and hence little star formation, that is usually elliptical. It contains only old stars, those stars are low mass (the massive stars having died off), and are consequently reddish.



You may be referring to the pinkish color often given to the H-alpha line in photos. This line is strongly emitted in the interstellar ionized hydrogen regions surrounding massive stars. Since the stars are massive, they only live a few tens of millions of years and are therefore characteristic of starburst regions.

Answer 2

the pictures in red show 'glowing hydrogen' according to this site...



http://www.utahskies.org/HST/Archives/galaxies.html



scroll down to the M51 picture.....



other starburst galaxy pics seems to show the hot new stars in blue.... several there, too.....enjoy that site... there's some amazing pictures there and good info, too!....

Answer 3

there is no such thing as a starburst galaxy. from the example you give, you appear to mean spiral galaxy.



the red is due to emissions from excited hydrogen. we see the same thing in our galaxy - i was looking at the orion nebula last night.



the colour is a matter of interpretation. emission nebulae generate light mainly at two wavelengths, one around 656 nanometers (red) and the other at 500 nanometers (greenish blue). our eyes are most sensitive to green, so if we see any colour at all in emission nebulae, we see them as green. colour photographic film was usually more sensitive to red, so emission nebulae photographed as red or pink.



when they started using ccd imaging they stuck with the "photographic" look, making emission nebulae red. if you image them with a single-shot colour ccd camera (like a dslr) they come out green or bluish-lavender.