Starship Asterisk*

Image Credit: ESA/Hubble & NASA, J. Dalcanton

---

This Picture of the Week features two interacting galaxies that are so intertwined, they have a collective name — Arp 91. This delicate galactic dance is taking place over 100 million light-years from Earth, and was captured by the NASA/ESA Hubble Space Telescope. The two galaxies comprising Arp 91 do have their own names: the lower galaxy, which in this image looks like a bright spot, is known as NGC 5953; and the ovoid galaxy to the upper right is NGC 5954. In reality, both of these galaxies are spiral galaxies, but their shapes appear very different because they are orientated differently with respect to Earth.

Arp 91 provides a particularly vivid example of galactic interaction. NGC 5954 is clearly being tugged towards NGC 5953 — it looks like it is extending one spiral arm downwards. It is the immense gravitational attraction of the two galaxies that is causing them to interact. Such gravitational interactions between galaxies are common, and are an important part of galactic evolution. Most astronomers nowadays believe that collisions between spiral galaxies lead to the formation of another type of galaxy, known as elliptical galaxies. These immensely energetic and massive collisions, however, happen on timescales that dwarf a human lifetime — they take place over hundreds of millions of years. So we should not expect Arp 91 to look any different over the course of our lifetimes!

Image Credit: Gideon Yoffe/ESO

---

A person’s welcoming silhouette is outlined against one of the Auxiliary Telescopes (ATs) at ESO's Paranal Observatory in the Atacama Desert of Chile, as the coalsack nebula waves its majestic hello as a dark smudge in the middle of the night sky.

This is one of ESO’s Very Large Telescope’s (VLT) four 1.8-metre Auxiliary Telescopes, which also includes four giant 8.2-metre cousins, the Unit Telescopes (UTs). Depending on the needs of each observing project, the ATs can be repositioned in up to 30 different observing locations along a system of tracks and act together as part of the VLT Interferometer. The interferometer combines the light the ATs or the UTs capture from celestial objects with a technique known as interferometry, allowing researchers to observe the cosmos in incredible detail.

The Coalsack Nebula is the most prominent dark nebula in the sky. In the Southern hemisphere, these dark clouds are more prominent than in the Northern Sky. Many cultures identify patterns in these dark nebulae, including the Mapuche people in Chile, who named the Coalsack Nebula pozoko, or “water well”.

As the Coalsack nebula is rich in dust, it absorbs and scatters most of the visible starlight behind it. However, these obscured stars can shine through dust when observed at infrared wavelengths, which can pass through dust almost unimpeded.

Image Credit: ESA/Hubble & NASA, O. Graur

---

NGC 4666 takes centre stage in this image from the NASA/ESA Hubble Space Telescope. This majestic spiral galaxy lies about 80 million light-years away in the constellation Virgo, and is undergoing a particularly intense episode of star formation. Astronomers refer to galaxies which are forming stars anomalously quickly as starburst galaxies. NGC 4666’s starburst is thought to be due to gravitational interactions with its unruly neighbours — including the nearby galaxy NGC 4668 and a dwarf galaxy.

NGC 4666’s burst of star formation is driving an unusual form of extreme galactic weather known as a superwind — a gigantic transfer of gas from the bright central heart of the galaxy out into space. This superwind is the result of driving winds from short-lived massive stars formed during NGC 4666’s starburst as well as spectacularly energetic supernova explosions. Two such supernova explosions have been seen in NGC 4666 within the last decade — one in 2014 and the other in 2019. The star which led to the 2019 supernova was recently determined to be 19 times as massive as our Sun!

At peak, supernovae are often the brightest sources of light in their galaxies, shining so bright that they can be seen clear across the Universe. The 2014 supernova in NGC 4666 is still active in this image, but more than 900 days after it peaked, the supernova has faded from its former glory and looks like just one more star in this busy galaxy.

Though the torrent of superheated gas emanating from NGC 4666 is truly vast in scale — extending for tens of thousands of light-years — it is invisible in this image. The superwind’s extremely high temperature makes it stand out as a luminous plume in x-ray or radio observations, but it doesn’t show up at the visible wavelengths imaged by Hubble’s Wide Field Camera 3.

starsurfer wrote: ↑Tue Oct 12, 2021 10:24 pm NGC 6726
https://www.astrobin.com/421158/
Copyright: Casey Good
wYYKHgMlfcPV_1824x0_RBO3NPph.jpg

Image Credit: Yuri Beletsky (LCO)/ESO

---

Cast against the background of a magnificent sunset above the Chajnantor Plateau in the Atacama Desert, the antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) keep watch over the skies. ALMA, of which ESO is a partner, is the largest ground-based astronomical project in existence.

The beautiful shades of orange and red that illuminate the sky as the Sun sets behind the horizon teach us an interesting physics lesson. As it passes through the Earth’s atmosphere, sunlight with shorter, bluer wavelengths gets scattered off the molecules of gas and dust in the air more than that with longer, redder wavelengths. As the Sun approaches the horizon, light that reaches the Earth’s surface takes a longer path through its atmosphere, and most of the blue part of its visible spectrum gets scattered away. The light that reaches our eyes dyes the sky around the setting Sun in stunning hues of red.

However, the beauty of the blood-red sky is invisible to ALMA. The 66 telescopes of ALMA don’t look at the sky in the visible part of the electromagnetic spectrum, as we do — they observe it in the millimetre and submillimetre wavelength range. Electromagnetic radiation of these wavelengths gets absorbed by water vapour in the Earth’s atmosphere, which is why telescopes used for submillimetre astronomy must be built in extremely dry locations, at high altitude — where atmospheric absorption doesn’t hinder their observations. At five kilometres above sea level, ALMA’s powerful antennas allow us to see the mysteries of the Universe in an extraordinary light.

Credit: ESA/Hubble and NASA, Dark Energy Survey, J. Dalcanton

---

This observation from the NASA/ESA Hubble Space Telescope showcases Arp 86, a peculiar pair of interacting galaxies which lies roughly 220 million light-years from Earth in the constellation Pegasus. Arp 86 is composed of the two galaxies NGC 7752 and NGC 7753 — NGC 7753 is the large spiral galaxy dominating this image, and NGC 7752 is its smaller companion. The diminutive companion galaxy almost appears to be attached to NGC 7753, and it is this peculiarity that has earned the designation “Arp 86” — signifying that the galaxy pair appears in the Atlas of Peculiar Galaxies compiled by the astronomer Halton Arp in 1966. The gravitational squabble between the two galaxies is doomed to end catastrophically for NGC 7752. It will eventually either be flung out into intergalactic space or be entirely engulfed by its far larger neighbour.

Hubble observed Arp 86 as part of a larger effort to understand the connections between young stars and the clouds of cold gas in which they form. Hubble gazed into star clusters and clouds of gas and dust in a variety of environments dotted throughout nearby galaxies. Combined with measurements from ALMA, a gigantic radio telescope perched high in the Chilean Andes, these Hubble observations provide a treasure trove of data for astronomers working to understand how stars are born.

These observations also helped sow the seeds of future research with an upcoming space telescope, the NASA/ESA James Webb Space Telescope (JWST). This telescope, due to launch later this year, will study star formation in dusty regions such as the galaxies of Arp 86.