![]() This raises questions about how M33 as a whole formed and what triggered Galaxy (M33), for example, contains young- and intermediate-age stars,Īnd has a star distribution that suggests the disk goes all the way to theĬenter. However, this view is challengedīy observations of some bulges. Halo, and, to a lesser extent, in the bulge. Outermost region of a galaxy, known as the galactic The original spherical shape lives on in the Spherical rotating mass of gas and dust, which gradually flattens out at Studying bulges can therefore tellĪstronomers about how galaxies formed and evolved.Īccording to one current theory, a spiral galaxy begins as a giant, roughly Stars that populate the bulgeĪre normally old, Population II objects, datingīack to their galaxy's earliest period. The bulgeĭiminishes in size relative to the galacticĭisk in the sequence of spirals Sa to Sd. The yoke, if such galaxies are imagined to resemble fried eggs. The galactic bulge is the spheroidal mass of stars that forms the central hub of spiral and lenticular galaxies – The formation of which is triggered by the movement through the disk of The arms are visible because of bright, massive stars, Have four or, in rare cases, three arms, in an arrangement that may complexĪnd fragmentary. Usually two armsĪre present that wrap around in a well-defined pattern however, some spirals That winds outward from the nucleus of a spiral The spiral galaxy NGC 2997 with well-defined spiral arms.Ī spiral arm is a curved feature, containing young stars, open Galaxy, the Sunflower Galaxy, the Black Eye Galaxy, the Sombreroįig 3. Other relatively close and well known spirals include the Triangulum Galaxy, the Whirlpool Our own Milky Way Galaxy, together with the Spirals represent 80% of the bright galaxies in regions outside of rich clusters (those having many members within a relatively compact region of space), there are no spirals with masses as low as those of many irregular Spiral structureĬan apparently exist only in disk galaxies above a certain size and, although In diameter, spirals range from about 10,000 to over 300,000 light-years,Īnd in mass from about one billion to 500 billion solar masses. Make up the class known as disk galaxies. The trend from a to d is toward decreasing brightness and mass. Into subtypes a (tightly wound arms, large bulge), b, c,Īnd (sometimes) d (loosely wound arms, small bulge). Spirals (type SB) and ordinary spirals (type S), each of which is divided There are two main types of spiral galaxy – barred As well as stars, theĭisk harbors extensive interstellar clouds of gas and dust. A spiral galaxy is a galaxy with a flattened disk and a nuclear bulge from which emanate
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