Hubble Space Telescope Got the GOODS!

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By Steven Spence

The Hubble Space Telescope has been conducting a deep-sky survey called “Great Observatories Origins Deep Survey North,” or GOODS North. The GOODS data has enabled astronomers to extend their view of galaxies back 150 million years further than any previously identified galaxy. The distant galaxy GN-Z11 was in the first generation of galaxies to form in the universe. Scientists estimate that it formed only 400 million years after the Big Bang. In fact, GN-Z11 formed at a time when the universe was mostly clouds of cold hydrogen gas.

“We managed to look back in time to measure the distance to a galaxy when the Universe was only three percent of its current age,” says Pascal Oesch of Yale University.

How Could Scientists Even Measure the Distance?

Astronomers have had to develop different methods to calculate the distance to objects. One method, called parallax, relies on careful observation and trigonometry to calculate the distance to another star. It’s a relatively simple method that involves taking two measurements at a known distance apart and comparing the angles. By measuring where a star is in December, and then again in June, scientists have a very large known distance to work with: the diameter of the Earth’s orbit. With precise timing and ultra-accurate measurements using the Hubble space telescope, it is possible to measure out to 10,000 light-years (a light-year is the distance that light travels in one Earth year: approximately 9.46 trillion km).

Hubble Space Telescope Got the GOODS!
Two examples of measuring parallax [By NASA/ESA, A.Feild/STScI [Public domain], via Wikimedia Commons]

Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to space. —Douglas Adams, The Hitchhiker’s Guide to the Galaxy

While 10,000 light-years sounds like a long distance, that’s only one-tenth the size of the Milky Way Galaxy. The Andromeda Galaxy, which is still visible to the naked eye, is 2.5 million light years away. Parallax can’t be used for such distances, so other methods had to be developed.

Being very resourceful, scientists came up with multiple methods involving the brightness and spectrum of a star. One standard measure involves using variable stars. Cepheid variable stars change in brightness in a known relationship to the pulsation period. This predictability makes it easy for scientists to determine how bright these stars are just by watching the period of their light curves. Cepheid variables can then be used comparatively to determine distances within the Milky Way and to “nearby” galaxies.

Hubble Space Telescope Got the GOODS!
RS Puppis as imaged by Hubble (HST) “This Hubble image shows RS Puppis, a type of variable star known as a Cepheid variable. As variable stars go, Cepheids have comparatively long periods— RS Puppis, for example, varies in brightness by almost a factor of five every 40 or so days. from [ESA/Hubble Public Domain]
A similar method—but one which uses a very rare event—is to wait for a supernova to explode. Since scientists also understand how the light curve of a supernova develops, they can determine how far away a galaxy is. Supernovas are spectacularly bright events, which makes it possible to see them much farther away than Cepheids can be seen.

Hubble Space Telescope Got the GOODS!
Distance 50 Million Light Years in the constellation Virgo.  CC BY 3.0, via Wikimedia Commons
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