Dwarf galaxies are simply galaxies relatively smaller in size and containing just a few hundred of thousand or just a few few billions of stars. This sounds like a lot, but when compared with larger galaxies such as ours (between 100 to 400 billion stars), the number appears somewhat smaller.
Being small and containing lesser stars, they have low mass. Due to the lack of abundance of stars, they have low luminosity too. They are very common actually.
In fact loads of them also orbit other galaxies. take ours for example. The Milky Way has at the very least, about 14 galaxies orbiting it.
Formation: Dwarf galaxies can be formed due to the collision between galaxies, which ejects streams of materials and dark matter, which cluster together to form smaller dwarf galaxies.
Another possibility can be that they were formed during the early stages the creation of the larger galaxies.
Types: Dwarf galaxies can be broadly categorized into five categories. Three of them are the same as the that of the 'regular' galaxies. But the last two get interesting.
Irregular dwarf galaxies:- They are irregular in their shape, just as the name sounds.
Elliptical dwarf galaxy:- They are ellipsoidal in shape, and smooth and featureless in appearance.
Spiral dwarf galaxy:- Again, as the name suggests, they are spiral in shape (similar to the Milky Way)
Blue Compact Dwarf:- They contain massive hot stars (the blue ones are the hottest). These hot ones make the star look blue in colour.
Ultra Faint Galaxies:- They have the least number of stars, making them look faint. They also have large amounts of dark matter.
Examples:
- Leo A: Irregular Dwarf galaxy
- Small Magellanic Cloud 2: Irregular Dwarf and a satellite of the milky way
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