Aurora over clouds

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APOD, auroras, Iceland

Image Credit & Copyright: Daniele Boffelli

Explanation: Auroras usually occur high above the clouds. The auroral glow is created when fast-moving particles ejected from the Sun impact the Earth’s magnetosphere, from which charged particles spiral along the Earth’s magnetic field to strike atoms and molecules high in the Earth’s atmosphere. An oxygen atom, for example, will glow in the green light commonly emitted by an aurora after being energized by such a collision. The lowest part of an aurora will typically occur at 100 kilometers up, while most clouds usually exist only below about 10 kilometers.

The relative heights of clouds and auroras are shown clearly in the featured picture from Dyrholaey, Iceland. There, a determined astrophotographer withstood high winds and initially overcast skies in an attempt to a capture aurora over a picturesque lighthouse, only to take, by chance, the featured picture along the way.

(This is the Nasa APOD from 24 November 2015. Click here for many more wonderful images. http://apod.nasa.gov/apod/astropix.html)

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The Black Hole in the Milky Way

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APOD, black holes, Milky Way, Sag-A*, Sagittarius A*

Praying one day as I read an APOD (NASA’s Astronomy Picture of the Day) about black holes, I asked the Lord a question. What are they for? “Trash compactors,” he replied. Hmmm. Here’s an APOD about our own Milky Way and it’s black hole, Sag-A*.

APOD 2012 November 2
Image Credit: NASA, JPL-Caltech, NuSTAR project

At the center of our Milky Way Galaxy, a mere 27,000 light-years away, lies a black hole with 4 million times the mass of the Sun. Fondly known as Sagittarius A* (pronounced A-star), the Milky Way’s black hole is fortunately mild-mannered compared to the central black holes in distant active galaxies, much more calmly consuming material around it. From time to time it does flare-up, though.

An outburst lasting several hours is captured in this series of premier X-ray images from the orbiting Nuclear Spectroscopic Telescope Array (NuSTAR). Launched last June 13 (2011), NuSTAR is the first to provide focused views of the area surrounding Sgr A* at X-ray energies higher than those accessible to Chandra and XMM observatories.

Spanning two days of NuSTAR observations, the flare sequence is illustrated in the panels at the far right. X-rays are generated in material heated to over 100 million degrees Celsius, accelerated to nearly the speed of light as it falls into the Miky Way’s central black hole.

The main inset X-ray image spans about 100 light-years. In it, the bright white region represents the hottest material closest to the black hole, while the pinkish cloud likely belongs to a nearby supernova remnant.

http://apod.nasa.gov/apod/ap121102.html