Caption: A true-color Hubble Space Telescope (HST) image of Abell S740 a rich galaxy cluster in the Abell catalogue of rich galaxy clusters.

Double-click for a high resolution image

Abell S740 is over 150 Mpc (450 Mly) away in Centaurus.

Just above the center of the image is the giant elliptical galaxy ESO 325-G004.

Its mass as 10**11 M_☉---which actually isn't all that big and I think this is only luminous matter and NOT counting the probably larger amount of dark matter.

The HST resolves thousands of globular clusters orbiting ESO 325-G004.

Globular clusters compact gravitationally bound star clusters containing thousands of stars.

In the image, the globular clusters are NOT readily identified, but they are probably some of the light points clustering near ESO 325-G004. Other points may be foreground or background objects.

The brightest foreground stars are easily identified as such---they have points. These stars are heavily overexposed because their apparent brightness is very high because of their nearness to us. The heavy overexposure brings out the diffraction patterns that their wave fronts create when cut by the aperture of the telescope. The stars are completely unresolved.

The extended galaxies are resolved, of course, and their diffraction patterns are relatively negligible.

How does one recognize elliptical galaxies?

First, in true color images (as ours is), ellipticals are pretty much all yellow.

This is because almost all their stars are relatively old stars and their are no hot young blue stars (e.g., OB stars). Star formation mostly turned billions of years ago in ellipticals.

And, of course, ellipticals don't have spiral arms which contain blue knots (due to hot young blue stars from recent star formation) and dark interstellar dust (which laces star formation regions).