Alternate Names

Nova Ophiuchi 1977, V2107 Ophiuchi, XN Oph77

Type

Stellar mass

Location

In the constellation Ophiuchus

Distance

27,000 light-years

Mass

5 to 7 times the mass of the Sun

Size

Diameter roughly 20 miles (35 km), equal to the size of a large city

Discovery Methods

• Detection of an accretion disk
• Measuring the motions of a companion star

H1705-25

In August of 1977, two orbiting telescopes detected a bright outburst of X-rays from the constellation Ophiuchus. The outburst was classified as an X-ray nova, which occurs when a disk of hot gas collapses onto a central neutron star or black hole, heating the gas and producing a torrent of X-rays and other energy.

Not until two decades later, though, did astronomers classify the source of the outburst as a black hole. Today, it is cataloged as H1705-25 for its coordinates in the sky.

Several studies have found that it is a binary system, with a small, cool star roughly one-third the mass of the Sun as a companion to the black hole. The two stars are so close together that they orbit each other once every 12.5 hours. At such close range, the black hole's powerful gravity pulls gas from its companion, encircling the black hole with an accretion disk.

The flow between the two stars is fairly slow and thin, so the accretion disk builds slowly. When it reaches a critical mass it collapses, triggering the outburst of energy that's known as an X-ray nova. Such a system can erupt many times, usually at intervals of a few decades.

From the characteristics of the companion star and its orbit around the black hole, astronomers have measured the black hole's mass at 5 to 7.5 times the mass of the Sun.

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This document was last modified: March 29, 2011.