Galaxies (natural sciences)

astronomers classify galaxies into three major categories, but here i'm gonna discuss 'bout Spiral Galaxies first okay :D

Spiral Galaxies


Like the Milky Way, other spiral galaxies also have a thin disk extending outward from a central bulge. The bulge itself merges smoothly into a halo that can extend to a radius of over 100,000 light-years. Together, the bulge and halo of a spiral galaxy make up its spheroidal component, so named because of its rounded shape. Although no clear boundary divides the pieces of the spheroidal component, astronomers usually consider stars within 10,000 light-years of the center to be members of the bulge and those outside this radius to be members of the bulge and those inside this radius to be members of the halo.

The disk component of a spiral galaxy slices directly through the halo and the bulge. The disk of a large spiral galaxy like the Milky Way can extend 50,000 light-years or more from the center.The disks of all spiral galaxies contain an interstellar medium of gas and dust, but the amounts and proportions of the interstellar medium of gas and dust, but the amounts and proportions of the interstellar medium in molecular, atomic, and ionized forms differ from one spiral galaxy to the next. Spiral galaxies with large bulges generally have less interstellar gas and dust than those with small bulges.

Not all galaxies have disks but do not appear to have spiral arms. These are called lenticular galaxies because they look lens-shaped when seen edge-on (lenticular means "lens shaped"). Although they look like spiral galaxies without arms, lenticular galaxies might more appropriately be considered an intermediate class between spirals and ellipticals because they tend to have less cool gas than normal spirals, but more than ellipticals.

Among large galaxies in the universe, most (75% to 85%) are spiral or lenticular. (Spiral and lenticular galaxies are much rarer among small galaxies.)


sumber : buku praktice for exercise TOEFL page 166


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