{"id":29867,"date":"2022-03-12T15:28:04","date_gmt":"2022-03-12T15:28:04","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=105637"},"modified":"2022-03-12T15:47:09","modified_gmt":"2022-03-12T15:47:09","slug":"what-is-an-accretion-disk-and-how-does-one-form","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/what-is-an-accretion-disk-and-how-does-one-form\/","title":{"rendered":"What is an accretion disk and how does one form?"},"content":{"rendered":"

<\/p>

By Paul Roche\n \t\t<\/p>

Published: Saturday, 12 March 2022 at 12:00 am<\/p>

\n\n

An accretion disk is a flattened, circular or elliptical structure that is formed when material falls towards a strong gravitational force, such as a star<\/a> or a black hole<\/a>.<\/p>\n

Accretion disks are found surrounding a variety of celestial bodies, from relatively small regions of a few thousand kilometres around white dwarfs and neutron stars<\/a>, to protoplanetary disks around very young stars.<\/span><\/p>\n

The biggest accretion disks, on the scale of the Solar System, are found surrounding the centres of active galaxies<\/a>.<\/span><\/p>\n

\""Radiation\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Radiation emanating from accretion disks is one way we can infer that a black hole exists. Credit: ESO\/L. Cal\u00e7ada<\/span><\/figcaption><\/div>\n

The fact that accretion disks exist over a huge range of scales and in apparently very different types of systems suggests some common physics behind their formation.<\/span><\/p>\n

Because angular momentum is always conserved, small rotations in any in-falling material are amplified as it collapses inwards towards the star or black hole.<\/span><\/p>\n

This is something like the motion of spinning ice skaters, who speed up as they bring their arms inwards.<\/span><\/p>\n

\""An\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
An image of a protoplanetary disk around star HL Tauri. The dark rings could indicate newly-forming planets in orbit, pushing aside dust as they go. Credit: ALMA (ESO\/NAOJ\/NRAO)<\/span><\/figcaption><\/div>\n

A \u2018centripetal force\u2019 \u2013 an inward-directed force on an orbiting body \u2013 then spreads the material out (sideways) to create the broad disk structure, while tidal forces pull it into an equatorial location.<\/span><\/p>\n

The combination of these two forces makes the shape of the disk wide, flat and thin.<\/span><\/p>\n

Within the accretion disk, colliding particles convert kinetic energy (the energy of motion) into heat and move inwards.<\/p>\n

\""The\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A disk of infalling matter allowed astronomers to photograph the black hole at the centre of galaxy M87. Credit: EHT Collaboration<\/span><\/figcaption><\/div>\n

As these particles move towards the central source, collisions occur more frequently due to the increased density of particles. As a result, they heat up and release X-rays.<\/p>\n

This radiation is an important tool for astronomers, as black holes can\u2019t be observed directly \u2013 not even light can escape the strength of their gravitiational pull.<\/span><\/p>\n

The X-rays released from accretion disks can be observed and used to locate black holes.<\/span><\/p><\/body><\/html>\n

","protected":false},"excerpt":{"rendered":"

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