APOD and General Astronomy Discussion Forum
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[img3="Credit: Petr Horálek / ESO"]https://cdn.eso.org/images/screen/potw1634a.jpg[/img3][hr][/hr]An unusual type of tourist is seen visiting ESO’s La Silla Observatory in this stunning wide-angle photograph taken in January 2015. Captured by ESO Photo Ambassador Petr Horálek, Comet C/2014 Q2 (Lovejoy) appears to streak across the sky (centre left of the image), sneaking past the two telescopes below: ESO’s 3.6-metre telescope (left) and the Swedish-ESO Submillimetre Telescope (SEST).
Like most comets, Comet Lovejoy enjoys a long, elliptical and eccentric orbit around the Sun. It entered the inner Solar System for the first recorded time in 2014, and reached perihelion — its closest approach to the Sun — on 30 January 2015. The distance between the Earth and the Sun is defined as 1 astronomical unit (au), or just under 150 million kilometres; Comet Lovejoy came within 1.29 au of our star, placing it between the orbits of Earth and Mars (1.52 au).
This image displays the characteristic soft green glow of the comet, produced as molecules of carbon are heated by the Sun. A tail of material splays out behind the comet’s nucleus, crafted by gas and dust blown from the comet by the wind of charged particles streaming out from the Sun.
This comet is actually the fifth to be discovered by its namesake, Terry Lovejoy, an amateur astronomer based in Queensland, Australia. Lovejoy previously discovered comets C/2007 E2, C/2007 K5, C/2011 W3, and C/2013 R1.
[c][attachment=0]potw1634a[1].jpg[/attachment][/c][hr][/hr]This image, courtesy of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys (ACS), captures the glow of distant stars within NGC 5264, a dwarf galaxy located just over 15 million light-years away in the constellation of Hydra (The Sea Serpent).
Dwarf galaxies like NGC 5264 typically possess around a billion stars — just one per cent of the number of stars found within the Milky Way. They are usually found orbiting other, larger, galaxies such as our own, and are thought to form from the material left over from the messy formation of their larger cosmic relatives.
NGC 5264 clearly possesses an irregular shape — unlike the more common spiral or elliptical galaxies — with knots of blue star formation. Astronomers believe that this is due to the gravitational interactions between NGC 5264 and other galaxies nearby. These past flirtations sparked the formation of new generations of stars, which now glow in bright shades of blue.
[img3="Credit: ESO / Babak Tafreshi (TWAN)"]https://cdn.eso.org/images/screen/potw1635a.jpg[/img3][hr][/hr]High up in the Chilean Atacama Desert, pioneering feats of human engineering collide with the majestic beauty of the natural world. This image shows ESO’s La Silla Observatory, where domes housing some of the most advanced astronomical instruments in the world sit beneath a sky shimmering with stars.
All of these stars belong to our home galaxy, the Milky Way. The Milky Way contains billions of stars, arranged in two strikingly different structures. The roughly spherical halo component, consisting mainly of older stars, appears in this image as the background of stars scattered across the sky. The second component is a thin disc made up of younger stars, gas and dust. We see this as a dense, bright, and visually stunning band running almost vertically across the sky. Pockets of dust block out the light from stars behind, giving the band a mottled appearance.
The bright concentration in the band of stars, located toward the top centre of this image, is the central region of the Milky Way. Here, astronomers have measured stars moving very much faster than anywhere else in our galaxy. This is taken as evidence for a supermassive black hole, some four million times the mass of the Sun, at the very centre of our galaxy. The black hole cannot be observed directly, but its presence can be inferred from the effect its enormous gravity has on the motions of these nearby stars.
[img3="Credit: ESA/Hubble & NASA"]https://cdn.spacetelescope.org/archives ... w1635a.jpg[/img3][hr][/hr]The closest star system to the Earth is the famous Alpha Centauri group. Located in the constellation of Centaurus (The Centaur), at a distance of 4.3 light-years, this system is made up of the binary formed by the stars Alpha Centauri A and Alpha Centauri B, plus the faint red dwarf Alpha Centauri C, also known as Proxima Centauri.
The NASA/ESA Hubble Space Telescope has given us this stunning view of the bright Alpha Centauri A (on the left) and Alpha Centauri B (on the right), flashing like huge cosmic headlamps in the dark. The image was captured by the Wide Field and Planetary Camera 2 (WFPC2). WFPC2 was Hubble’s most used instrument for the first 13 years of the space telescope’s life, being replaced in 2009 by WFC3 during Servicing Mission 4. This portrait of Alpha Centauri was produced by observations carried out at optical and near-infrared wavelengths.
Compared to the Sun, Alpha Centauri A is of the same stellar type G2, and slightly bigger, while Alpha Centauri B, a K1-type star, is slightly smaller. They orbit a common centre of gravity once every 80 years, with a minimum distance of about 11 times the distance between the Earth and the Sun. Because these two stars are, together with their sibling Proxima Centauri, the closest to Earth, they are among the best studied by astronomers. And they are also among the prime targets in the hunt for habitable exoplanets. Using the HARPS instrument astronomers already discovered a planet orbiting Alpha Centauri B. 24 August 2016 astronomers announced the discovery of an Earth-like planet in the habitable zone orbiting the star Proxima Centauri.
