Messier 51 (The Whirlpool Galaxy)
Credits: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)
Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
NGC 4038-4039 (The Antennae Galaxies)
Credits: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration
Our stellar count has jumped and we’re not done quite yet!
Nearby Spica, about a hand width to the southwest, is a bright “sail” of four stars, making up the constellation of Corvus, the Crow. Looking a smidge to the west of the top right star—Gienah Ghurab—a medium-size telescope can resolve a fuzzy heart-shaped object. This is NGC 4038, or the Antennae Galaxies. The heart shape has been resolved by the Hubble Space Telescope into not one, but two galaxies, in the middle of a galactic collision! These two galaxies first started interacting a few hundred million years ago and now, millions of years later, we’re seeing the middle stage of this cosmic crash. The merger of these two galaxies is easy for us to spot—even at over 50 million light-years away—because as the gas and dust collide, regions collapse together into super star clusters, giving birth to billions of new stars. Looking at images from the Hubble Space Telescope, astronomers are able to estimate that even just the small bright spots are young clusters containing tens of thousands of stars.
As we observe the universe with better and better equipment we’ve found that galactic interactions are incredibly common. If you stick around long enough (just five billion years, give or take a few hundred million), the Milkyway and Andromeda galaxies will merge into a new super galaxy, jump-starting a new reign of star formation all around us. That’s not surprising, as we’ve also found that galaxies are incredibly common. Long-term deep sky surveys like the 2dF Galaxy Redshift Survey or the Two Micron All Sky Survey have enabled us to gain an unprecedented understanding of what is out there. The most detailed survey has been going on for almost 20 years with the Sloan Digital Sky Survey at Apache Point Observatory in New Mexico. This cosmic program uses a 2.5-meter telescope that has mapped over a third of the entire sky, finding over 1 billion galaxies, and allowing scientists to create the most detailed three-dimensional maps of the universe ever made. With SDSS data and other observations combined, our observable universe is estimated to contain upwards of two trillion galaxies. Each of these galaxies is a little bit different, but based on those observations, we can say that, on average, a galaxy has 100 billion stars in it.
So how many stars are there? Multiple 2 trillion galaxies by 100 billion stars per galaxy… 2,000,000,000,000 x 100,000,000,000 = 200,000,000,000,000,000,000,000 stars! (That’s 2e23, or 2 with 23 zeros after it!)
The number of stars is almost unimaginable, but for some comparison, the total number of grains of sand on the Earth is estimated to be 7,500,000,000,000,000,000 or 7.5 with 18 zeros after it. With rough estimates, there are more stars in the universe than grains of sand on the Earth. On the other hand, if you’re now feeling a little lost on the beach, contemplating the grand infinity of the universe, take a moment and drink some fresh water, and think—just a teaspoon of that water has the same number of molecules as there are stars in the sky.