The Milky Way. Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and at least ten times as much dark matter as all the stars and gas put together. And it’s all held together by gravity. Like more than two-thirds of the known galaxies, the Milky Way has a spiral shape. By convention, the standard reference epoch is J2000.0, which refers to the mean equator and equinox of year 2000, nominally January 1st 12:00 hours Universal Time (UT). The "J" means Julian year, which is 365.25 days long. Only the 26,000-year precession part of the whole precession/nutation effect is considered, defining the mean equator and Universe 2022, 8, 111 4 of 30 massive star clusters in the Milky Way and in external galaxies, both from an observational and a theoretical perspective. Krumholz et al. [49] provided a recent 7.1. When e = 0, the ellipse is a circle. The area of an ellipse is given by A = π a b, where b is half the short axis. If you know the axes of Earth’s orbit and the area Earth sweeps out in a given period of time, you can calculate the fraction of the year that has elapsed. .

milky way revolution time