Starship Asterisk*

[img3="Credit: ESO/VLT/UVES
Acknowledgement: V. D’Odorico (Osservatorio Astronomico di Trieste)"]https://cdn.eso.org/images/screen/potw1649a.jpg[/img3][hr][/hr]

The most massive galaxies in the Universe host supermassive black holes at their centres. These truly colossal black holes chew up surrounding material at astonishing rates, expelling huge amounts of radiation as they do so and glowing as some of the brightest objects in the known Universe! Despite their incredible distances from Earth, the regions surrounding these black holes shine so brightly that their appearance is similar to that of stars in our own galaxy, the Milky Way.

Some of these objects, known as quasi-stellar-objects or quasars for short, are useful tools to help us better understand the cosmos. Because they lie so far away, there is plenty of intervening space between our telescopes and a target quasar. This space is not empty; it is filled with intergalactic medium, which mostly comprises clouds of gas — mainly hydrogen and helium, but also with hints of other elements — that absorb light from more distant sources and prevent it from reaching us. The light emitted from bright quasars has to travel through these clouds on its journey to us, and so is partly absorbed.

This spectrum, taken by the UVES instrument mounted on ESO’s Very Large Telescope in Chile, shows the light from a quasar, catchily named HE 0940-1050, after it has travelled through such clouds. The vertical lines are tell-tale signs of absorption — they show where light has been absorbed by the gas in the intergalactic medium and thus removed from the original quasar spectrum. The intensity of the lines is linked to the amount of material which is crossed by the light. By analysing these lines, astronomers can infer all sorts of information about the material from which the clouds are made. The exceptional value of this particular spectrum is in the very faint lines which are the faintest ever observed in a quasar spectrum.

[img3="Credit: ESA/Hubble & NASA"]https://cdn.spacetelescope.org/archives ... w1649a.jpg[/img3][hr][/hr]

The constellation of Virgo (The Virgin) is especially rich in galaxies, due in part to the presence of a massive and gravitationally-bound collection of over 1300 galaxies called the Virgo Cluster. One particular member of this cosmic community, NGC 4388, is captured in this image, as seen by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

Located some 60 million light-years away, NGC 4388 is experiencing some of the less desirable effects that come with belonging to such a massive galaxy cluster. It is undergoing a transformation, and has taken on a somewhat confused identity.

While the galaxy’s outskirts appear smooth and featureless, a classic feature of an elliptical galaxy, its centre displays remarkable dust lanes constrained within two symmetric spiral arms, which emerge from the galaxy’s glowing core — one of the obvious features of a spiral galaxy. Within the arms, speckles of bright blue mark the locations of young stars, indicating that NGC 4388 has hosted recent bursts of star formation.

Despite the mixed messages, NGC 4388 is classified as a spiral galaxy. Its unusual combination of features are thought to have been caused by interactions between NGC 4388 and the Virgo Cluster. Gravitational interactions — from glancing blows to head-on collisions, tidal influencing, mergers, and galactic cannibalism — can be devastating to galaxies. While some may be lucky enough to simply suffer a distorted spiral arm or newly-triggered wave of star formation, others see their structure and contents completely and irrevocably altered.

Ann wrote:The colors of this galaxy are interesting.

The B-V of NGC 4388 is 0.740. This is quite unremarkable and suggests that the galaxy contains decidedly non-negligible numbers of bright young stars, but it has a very well-established old population, too.

The U-B is 0.280. That is relatively red, and suggests that the young stars of this galaxy are blue and aging rather than piping hot, ultraviolet and all but newborn.

But the far infrared magnitude is interesting. NGC 4388 is bright in far infrared, almost one and a half magnitudes brighter than in B magnitude. This suggests that the galaxy is dust-reddened, and contains more OB stars than the color indexes would indicate.

Ann

starsurfer wrote:NGC 4639
http://www.spacetelescope.org/images/potw1541a/
Copyright: ESA/Hubble & NASA
potw1541a.jpg

[img3="Credit: ESA/Hubble & NASA"]https://cdn.spacetelescope.org/archives ... w1650a.jpg[/img3][hr][/hr]

In 1900, astronomer Joseph Lunt made a discovery: Peering through a telescope at Cape Town Observatory, the British–South African scientist spotted this beautiful sight in the southern constellation of Grus (The Crane): a barred spiral galaxy now named IC 5201.

Over a century later, the galaxy is still of interest to astronomers. For this image, the NASA/ESA Hubble Space Telescope used its Advanced Camera for Surveys (ACS) to produce a beautiful and intricate image of the galaxy. Hubble’s ACS can resolve individual stars within other galaxies, making it an invaluable tool to explore how various populations of stars have sprung to life, evolved, and died throughout the cosmos.

IC 5201 sits over 40 million light-years away from us. As with two thirds of all the spirals we see in the Universe — including the Milky Way, the galaxy has a bar of stars slicing through its centre.