Page 45 - Curriculum Visions Dynamic Book

Types of galaxies
One important step to finding out about galaxies is to look for patterns among them. Once you have done this, you can group galaxies into similar types. This is called classification.
Edwin Hubble was the first person to classify galaxies (pages 42–43). The first type he called “irregulars,” having no apparent pattern of bright stars inside them. The light from them tends to be blue-white, which is an indication of young, hot stars. It is likely that these galaxies are creating huge numbers of stars, but that their development has not yet reached a stage where they show any organization. Our neighboring galaxies, the Large and Small Magellanic Clouds (see page 51), are irregular galaxies of this kind.
Besides the irregular galaxies there are elliptical galaxies and spiral galaxies. Elliptical galaxies tend to be football shaped (oval). Spiral galaxies are flat and disk shaped. Some spiral galaxies have curved arms; others have bars of stars and are known as barred spirals.
The ellipticals and spirals account for most of the galaxies in the Universe.
Radio galaxies
Although stars are all amazingly good at converting
large amounts of matter into energy, the most powerful ones do not necessarily convert matter into light
energy. When scientists study the Universe in radio or X-ray wavelengths rather than in visible light, they find it studded with galaxies that are completely invisible to an ordinary telescope. They are called radio galaxies and were once referred to as radio stars (see page 36).
 This is the Whirlpool Galaxy (catalogue numbers NGC 5194 and M51). It can be seen with smaller telescopes.
You can see numerous shining clusters of young and energetic stars. The bright clusters are surrounded by glowing hydrogen gas.
Along the spiral arms dust “spurs” are seen branching out almost perpendicular to the main spiral arms. There is a dust disk in the nucleus, which may provide fuel for a black hole.
elliptical galaxy A galaxy that has an oval shape rather like a football, and that has no spiral arms.
radio galaxy A galaxy that gives out radio waves of enormous power.
spiral galaxy A galaxy that has a core of stars at the center of long curved arms made of even more stars arranged in a spiral shape.
 This is the galaxy Centaurus
A (NGC 5128) in the southern sky. Its appearance is due to an opaque dust lane that covers the central part of the galaxy. This dust is likely to be the remains of a cosmic merger between a giant elliptical galaxy and a smaller spiral galaxy full of dust.
Centaurus A is one of the brightest radio sources in the
sky. It is 11 million light-years away and is also the nearest radio galaxy. The radio emission is probably due to material being sucked into a massive black hole.
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