A spiral in Andromeda: Not to be confused with our neighbouring Andromeda Galaxy, the Andromeda constellation is one of the 88 modern constellations. More importantly for this image, it is home to the pictured NGC 7640.
Many different classifications are used to identify galaxies by shape and structure — NGC 7640 is a barred spiral type. These are recognisable by their spiral arms, which fan out not from a circular core, but from an elongated bar cutting through the galaxy’s centre. Our home galaxy, the Milky Way, is also a barred spiral galaxy. NGC 7640 might not look much like a spiral in this image, but this is due to the orientation of the galaxy with respect to Earth — or to Hubble, which acted as photographer in this case! We often do not see galaxies face on, which can make features such as spiral arms less obvious. 
There is evidence that NGC 7640 has experienced some kind of interaction in its past. Galaxies contain vast amounts of mass, and therefore affect one another via gravity. Sometimes these interactions can be mild, and sometimes hugely dramatic, with two or more colliding and merging into a new, bigger galaxy. Understanding the history of a galaxy, and what interactions it has experienced, helps astronomers to improve their understanding of how galaxies — and the stars within them — form.

Name: NGC 7640
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Andromeda
Distance: 30 million light years
Release date: 6 February 2017, 06:00
Credit: ESA/Hubble & NASA

Sidekick or star of the show?: This image was captured by the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys (ACS), a highly efficient wide-field camera covering the optical and near-infrared parts of the spectrum. While this lovely image contains hundreds of distant stars and galaxies, one vital thing is missing — the object Hubble was actually studying at the time! This is not because the target has disappeared. The ACS actually uses two detectors: the first captures the object being studied — in this case an open star cluster known as NGC 299 — while the other detector images the patch of space just ‘beneath’ it. This is what can be seen here. Technically, this picture is merely a sidekick of the actual object of interest — but space is bursting with activity, and this field of bright celestial bodies offers plenty of interest on its own. It may initially seem to show just stars, but a closer look reveals many of these tiny objects to be galaxies. The spiral galaxies have arms curving out from a bright centre. The fuzzier, less clearly shaped galaxies might be ellipticals. Some of these galaxies contain millions and millions of stars, but are so distant that all of their starry residents are contained within just a small pinprick of light that appears to be the  same size as a single star! The bright blue dots are very hot stars, sometimes distorted into crosses by the struts supporting Hubble’s secondary mirror. The redder dots are cooler stars, possibly in the red giant phase when a dying star cools and expands.

Name: Field stars
Type: Milky Way : Star
Category: Stars
Constellation: Tucana
Release date: 13 February 2017, 06:00
Credit: ESA/Hubble & NASA

Astronomical cooperation: The events surrounding the Big Bang were so cataclysmic that they left an indelible imprint on the fabric of the cosmos. We can detect these scars today by observing the oldest light in the Universe. As it was created nearly 14 billion years ago, this light — which exists now as weak microwave radiation and is thus named the cosmic microwave background (CMB) — has now expanded to permeate the entire cosmos, filling it with detectable photons. The CMB can be used to probe the cosmos via something known as the Sunyaev-Zel’dovich (SZ) effect, which was first observed over 30 years ago. We detect the CMB here on Earth when its constituent microwave photons travel to us through space. On their journey to us, they can pass through galaxy clusters that contain high-energy electrons. These electrons give the photons a tiny boost of energy. Detecting these boosted photons through our telescopes is challenging but important — they can help astronomers to understand some of the fundamental properties of the Universe, such as the location and distribution of dense galaxy clusters. The NASA/ESA Hubble Space Telescope observed one of most massive known galaxy clusters, RX J1347.5–1145, seen in this Picture of the Week, as part of the Cluster Lensing And Supernova survey with Hubble (CLASH). This observation of the cluster, 5 billion light-years from Earth, helped the  Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study the cosmic microwave background using the thermal Sunyaev-Zel’dovich effect. The observations made with ALMA are visible as the blue-purple hues.

Name: RX J1347.5–1145
Type: Early Universe : Galaxy : Grouping : ClusterEarly Universe : Cosmology : Morphology : Cosmic Background
Category: Cosmology
Constellation: Virgo
Distance: z=0.451 (redshift)
Release date: 20 February 2017, 06:00
Credit: ESA/Hubble & NASA, T. Kitayama (Toho University, Japan)/ESA/Hubble & NASA

A CLASH of galaxies: This cluster of galaxies, RX J1347.5–1145, was observed by the NASA/ESA Hubble Space Telescope as part of the Cluster Lensing And Supernova survey with Hubble (CLASH). The cluster is one of most massive known galaxy clusters in the Universe.

Name: RX J1347.5–1145
Type: Early Universe : Galaxy : Grouping : Cluster
Category: CosmologyGalaxies
Constellation: Virgo
Distance: z=0.451 (redshift)
Release date: 20 February 2017, 06:00
Credit: ESA/Hubble, NASA

A remarkable galactic hybrid: This NASA/ESA Hubble Space Telescope image showcases the remarkable galaxy UGC 12591. Classified as an S0/Sa galaxy, UGC 12591 sits somewhere between a lenticular and a spiral. It lies just under 400 million light-years away from us in the westernmost region of the Pisces–Perseus Supercluster, a long chain of galaxy clusters that stretches out for 250 million light-years — one of the largest known structures in the cosmos. The galaxy itself is also extraordinary: it is incredibly massive. The galaxy and its halo together contain several hundred billion times the mass of the Sun; four times the mass of the Milky Way. It also whirls round extremely quickly, rotating at speeds of up to 1.8 million kilometres per hour! Observations with Hubble are helping astronomers to understand the mass of UGC 1259, and to determine whether the galaxy simply formed and grew slowly over time, or whether it might have grown unusually massive by colliding and merging with another large galaxy at some point in its past.

Name: LEDA 71392, UGC 12591
Type: Local Universe : Galaxy : Type : SpiralLocal Universe : Galaxy : Type : Lenticular
Category: Galaxies
Constellation: Pegasus
Release date: 27 February 2017, 06:00
Credit: ESA/Hubble & NASA