Image Credit: NASA, ESA, A. Riess (STScI/JHU), L. Macri (Texas A & M Univ.) et al., Hubble Heritage (STScI/AURA)
NGC 5584 is a face-on spiral galaxy, more than 50,000 light-years across, that resides 72 million light-years away in the constellation Virgo. Its winding spiral arms are loaded with luminous young star clusters and thin dark dust lanes which appear to be flowing from the yellowish core, where older stars reside.
Among the galaxy’s myriad stars are 250 stars that vary in brightness and are classified as Cepheids. These are brilliant pulsating stars with a remarkable property — once the time it takes a Cepheid to brighten and fade is known, then it is possible to find how bright it actually is. When this information is combined with a measurement of how bright the star appears it is easy to work out how far away the star actually lies. This method is the most accurate and effective way to measure the distances to most nearby galaxies.
In addition to these Cepheids, NGC 5584 was also the site of SN 2007af, a type Ia supernova. These dramatic explosions of white dwarf stars are used as reference beacons for mapping the expansion, and acceleration, of the more remote Universe so this galaxy is a very valuable link between the two distance scales.
Once astronomers know accurate distances to galaxies near and far, they can determine the universe’s expansion rate. By studying many Cepheids in 8 galaxies (was one of them was NGC 5584) the team has been able to refine our knowledge of this expansion rate, expressed as a number known as Hubble’s constant, to an accuracy of 3.3 percent.
The value for the expansion rate is 73.8 kilometers per second per megaparsec. It means that for every additional million parsecs (3.26 million light-years) a galaxy is from Earth, the galaxy appears to be traveling 73.8 kilometers per second faster away from us.
Every decrease in uncertainty of the universe’s expansion rate helps solidify our understanding of its cosmic ingredients. Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and also helps astronomers tighten up a number of other cosmic properties, including the universe’s shape and its roster of neutrinos, ghostly particles that filled the early universe.
The reddish dots sprinkled throughout the image are largely background galaxies.