The Galaxy Evolution Explorer's ultraviolet eyes have captured a globular
star cluster, called NGC 362, in our own Milky Way galaxy. In this new
image, the cluster appears next to stars from a more distant neighboring
galaxy, known as the Small Magellanic Cloud.
Globular clusters are densely packed bunches of old stars scattered in
galaxies throughout the universe. NGC 362, located 30,000 light-years
away, can be spotted as the dense collection of mostly yellow-tinted stars
surrounding a large white-yellow spot toward the top-right of this image.
The white spot is actually the core of the cluster, which is made up of
stars so closely packed together that the Galaxy Evolution Explorer cannot
see them individually.
The light blue dots surrounding the cluster core are called extreme
horizontal branch stars. These stars used to be very similar to our sun
and are nearing the end of their lives. They are very hot, with
temperatures reaching up to about four times that of the surface of our
sun (25,000 Kelvin or 45,500 degrees Fahrenheit).
A star like our sun spends most of its life fusing hydrogen atoms in its
core into helium. When the star runs out of hydrogen in its core, its
outer envelope will expand. The star then becomes a red giant, which burns
hydrogen in a shell surrounding its inner core. Throughout its life as a
red giant, the star loses a lot of mass, then begins to burn helium at its
core. Some stars will have lost so much mass at the end of this process,
up to 85 percent of their envelopes, that most of the envelope is gone.
What is left is a very hot ultraviolet-bright core, or extreme horizontal
branch star.
Blue dots scattered throughout the image are hot, young stars in the Small
Magellanic Cloud, a satellite galaxy of the Milky Way located
approximately 200,000 light-years away. The stars in this galaxy are much
brighter intrinsically than extreme horizontal branch stars, but they
appear just as bright because they are farther away. The blue stars in the
Small Magellanic Cloud are only about a few tens of millions of years old,
much younger than the approximately 10-million-year-old stars in NGC 362.
Because NGC 362 sits on the northern edge of the Small Magellanic Cloud
galaxy, the blue stars are denser toward the south, or bottom, of the
image.
Some of the yellow spots in this image are stars in the Milky Way galaxy
that are along this line of sight. Astronomers believe that some of the
other spots, particularly those closer to NGC 362, might actually be a
relatively ultraviolet-dim family of stars called "blue stragglers." These
stars are formed from collisions or close encounters between two closely
orbiting stars in a globular cluster.
This image is a false-color composite, where light detected by the Galaxy
Evolution Explorer's far-ultraviolet detector is colored blue, and light
from the telescope's near-ultraviolet detector is red.