This spiral galaxy Not only is it beautiful – although it certainly is. The star system is also a standard for measuring the enormity of space.
Spiral galaxy UCG 9391 takes center stage in this stunning image from the Hubble Space Telescope, but the most dramatic part of the scene is the distance between the stars in the foreground and the galaxies in the background. Bright diffraction spikes define nearby luminous stars, while astoundingly distant galaxies range from tiny spirals to faint twitches of light. Imagine the enormity of a galaxy of hundreds of billions of stars. How far should a galaxy appear as a single point of light in the distance?
The galaxy at the center of this image helps astronomers answer this question more precisely.
Astronomers can measure the distance directly to nearby stars – within a few hundred light years from Earth – using some fairly simple math: measure the angle between the star and the horizon and repeat the measurement 6 months later when the Earth has completed half an orbit around the Sun. (We know the diameter of the Earth’s orbit, which is roughly 300 million kilometers. In other words, you have a side length and two angles in a triangle. The rest is just math.)
For stars that are slightly more distant, color is a good guide. The color of a star’s light is coordinated with how bright the star actually is, which is what astronomers call its luminosity. But how bright a star is depends on how far away it is, so astronomers can measure the color of the star and how bright it is from Earth, then use the difference to work out the distance to the star.
Astronomers use a different version of the luminosity-versus-luminosity trick to measure the distance to more distant objects, such as distant galaxies. But in order to do so, they need things – called standard candles – whose luminosity astronomers know or can calculate. Two of the best standard candles are Type 1a supernovae (which occur when a white dwarf star, in a binary star system, gains so much material from its partner that it explodes) as well as stars called Qiped variables. The variable cepheid star brightens and dims at regular intervals, and the length of those periods is directly related to the star’s light.
Recently, astronomers studying the spiral galaxy UGC 9391, pictured in this new Hubble image, were able to compare distances they calculated using supernovae and Cepheid variables. By detecting shifts in results from the two methods, astronomers can work to make cosmic distance measurements more accurate.
Knowing the exact distance between two objects in space is very important, as it can help physicists know how fast the universe is expanding. Most importantly, it can tell us what fate awaits the universe and everything in it. We know that the universe is still expanding 13.8 billion years after the Big Bang, but how fast that happens, and whether it’s accelerating or slowing down, will tell us many important details. First, it will reveal if we have enough thermal energy for the universe to continue expanding (heat death), if the universe is about to tear itself apart, or if it will re-enter itself instead of continuing to expand (big crunch).
And thanks to a dwarf galaxy about 130 million light-years away, we’re one step closer to getting answers.