How We Know How Many Galaxies Are In The Universe, Thanks To Hubble

Ethan Siegel 
As the Hubble Space Telescope celebrates its 25th anniversary, it’s easy to take for granted all we’ve learned thanks to its incredible discoveries. But don’t forget that when it was first launched, the spherical aberration in the primary mirror caused all the images it was returning to be blurry and distorted. What was supposed to be the flagship NASA science mission of the 1990s was shaping up to be a catastrophe. There were only two choices: fly to space and service the telescope, or simply accept the woefully inadequate equipment.

The decision was made to fly the space shuttle to dock with Hubble, and fix the mirror. When the telescope was serviced in December of 1993, the Corrective Optics Space Telescope Axial Replacement (COSTAR) and the Wide Field and Planetary Camera 2 (WFPC2) were both installed, not only fixing the optics but enhancing the camera technology on board. The result was a sharpening of the resolution by a factor of more than a hundred from what was previously available through Hubble.

Image credit: NASA, STScI, of the nucleus of the galaxy M100 before (L) and after (R) the servicing mission.

With the greatest-ever optical systems and observing power at its disposal, a very unusual proposal was greenlit. A team was going to point Hubble at a patch of sky without an observing target. Rather than observe a planet, star, nebula, cluster or galaxy, Hubble was going to observe the black void of empty space. By pointing at a patch of sky with nothing in it:

• no bright stars,
• no gas clouds,
• no nebulae,
• no known galaxies,
• no clusters,

astronomers were hoping to find out whether empty space was truly empty, or whether, as the Big Bang and the cosmological principle predicted, a plethora of galaxies from the distant Universe would be revealed to us.

Image credit: NASA / Digital Sky Survey, STScI.

The Hubble Space Telescope orbits the Earth approximately once every 90 minutes, or 18 times per day. Over a total of 342 cycles (where each cycle is a half-an-orbit), it images this exact patch of sky, a patch where only maybe five or six very faint stars within our galaxy lived. It pointed away from the galactic plane, out into intergalactic space, at a blank location known to be empty to the limits of our largest and most advanced ground based telescopes. And after a total of nearly ten days‘ worth of light had been gathered in multiple different color filters, they composited the image and released it to the world. Here’s what it saw.

Image credit: R. Williams (STScI), the Hubble Deep Field Team and NASA.

In this tiny region of space, where zero known galaxies were seen previously, we now had discovered around three thousand. With the exception of about five or six points showing diffraction spikes (the “pointy” features), which are stars within the Milky Way, every single point of light in this image is a galaxy. This is the Hubble Deep Field. More recently, as of 2012, an even deeper version was created, using a more advanced set of cameras and optics, and a total of twenty-three days worth of observing: the Hubble eXtreme Deep Field (XDF).

Image credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team.

This latter image, consisting of a region of space barely a thousandth of a square degree on the sky – so small it would take thirty-two-million of them to fill the entire sky — contains a whopping 5,500 galaxies, the most distant of which have had their light traveling towards us for some 13 billion years, or more than 90% the present age of the Universe. Extrapolating this over the entire sky, we find that there are 170 billion galaxies in the observable Universe, and that’s just a lower limit. Given that there’s an astonishing amount of Universe out there that Hubble cannot yet see — and that it can only see the brightest of the most distant galaxies — we may yet find closer to 10^12 (or a trillion) galaxies within our visible Universe.

And given that there are hundreds of billions of star within our own galaxy, that teaches us there are some 10^23 stars in the Universe, each one with their own unique story, history, planetary systems, and just possibly a chance at life. So on your 25 anniversary, for all that you’ve done, thank you, Hubble.

Ethan Siegel is the writer and founder of Starts With A Bang, and professor of physics at Lewis & Clark College in Portland, OR. His first book, Beyond the Galaxy, is due out later this year.