File:Three Steps to the Hubble Constant (40382081612).png

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Description	These Hubble Space Telescope images showcase 2 of the 19 galaxies analyzed in a project to improve the precision of the universe's expansion rate, a value known as the Hubble constant. The color-composite images show NGC 3972 (left) and NGC 1015 (right), located 65 million light-years and 118 million light-years, respectively, from Earth. The yellow circles in each galaxy represent the locations of pulsating stars called Cepheid variables. These stars blink at a rate matched closely by their intrinsic brightness, making them ideal cosmic lighthouses for measuring accurate distances to relatively nearby galaxies. Another reliable milepost marker is a special class of exploding star called a Type Ia supernova. All of these supernovae peak at the same brightness and are brilliant enough to be seen over relatively longer distances. The small cross-shaped feature in each galaxy denotes the location of a Type Ia supernova. Astronomers search for Cepheid variables in nearby galaxies containing a Type Ia supernova so they can compare the true brightness of both types of stars. That brightness information is used to calibrate the luminosity of Type Ia supernovae in far-flung galaxies so that astronomers can calculate the galaxies' distances from Earth. Once astronomers know accurate distances to galaxies near and far, they can determine and refine the universe's expansion rate. The observations for NGC 3972 were taken in 2015; for NGC 1015 in 2013. Both galaxies were observed by Hubble's Wide Field Camera 3.
Date	22 February 2018, 00:00
Source	Three Steps to the Hubble Constant
Author	NASA Hubble Space Telescope

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This image was originally posted to Flickr by NASA Hubble at https://flickr.com/photos/144614754@N02/40382081612 (archive). It was reviewed on 13 July 2018 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

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Source	STScI
Headline	Hubble Helps Astronomers Measure Accurate Distances to Galaxies
Credit/Provider	NASA, ESA, A. Riess (STScI/JHU)
Short title	Three Steps to the Hubble Constant
Image title	These Hubble Space Telescope images showcase 2 of the 19 galaxies analyzed in a project to improve the precision of the universe's expansion rate, a value known as the Hubble constant.The color-composite images show NGC 3972 (left) and NGC 1015 (right), located 65 million light-years and 118 million light-years, respectively, from Earth. The yellow circles in each galaxy represent the locations of pulsating stars called Cepheid variables. These stars blink at a rate matched closely by their intrinsic brightness, making them ideal cosmic lighthouses for measuring accurate distances to relatively nearby galaxies.Another reliable milepost marker is a special class of exploding star called a Type Ia supernova. All of these supernovae peak at the same brightness and are brilliant enough to be seen over relatively longer distances. The small cross-shaped feature in each galaxy denotes the location of a Type Ia supernova.Astronomers search for Cepheid variables in nearby galaxies containing a Type Ia supernova so they can compare the true brightness of both types of stars. That brightness information is used to calibrate the luminosity of Type Ia supernovae in far-flung galaxies so that astronomers can calculate the galaxies' distances from Earth. Once astronomers know accurate distances to galaxies near and far, they can determine and refine the universe's expansion rate.The observations for NGC 3972 were taken in 2015; for NGC 1015 in 2013. Both galaxies were observed by Hubble's Wide Field Camera 3.
Author	Space Telescope Science Institute Office of Public Outreach
Usage terms	http://hubblesite.org/copyright/
Date and time of data generation	00:00, 22 February 2018
Date and time of digitizing	12:37, 26 January 2018
Date metadata was last modified	07:00, 15 February 2018
Unique ID of original document	xmp.did:553e8cae-31cd-4afe-a7ef-bbc818727f41
Keywords	NGC 1015
Contact information	outreach@stsci.edu http://hubblesite.org 3700 San Martin Drive Baltimore, MD, 21218 USA 410-338-4444
Horizontal resolution	28.34 dpc
Vertical resolution	28.34 dpc
File change date and time	11:39, 15 February 2018

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