The westernmost star in Orion's belt is known officially as
Delta Orionis. Delta Orionis is a small stellar group with three components and
five stars in total: Delta Ori A, B, C. Both Delta Ori B and C are single stars and may give off small amounts of X-rays. Delta Ori A, on the other hand, has been detected as a strong X-ray source and is itself a
triple star system.
In Delta Ori A, two closely separated stars orbit around each other every 5.7 days, while a third star orbits this pair with a period of over 400 years. The more massive, or primary, star in the closely-separated stellar pair weighs about 25 times the mass of the Sun, whereas the less massive, or secondary star, weighs about 10 times the mass of the Sun. The chance alignment of this pair of stars allows one star to pass in front of the other during every orbit from the vantage point of Earth. This special class of star system is known as an "
eclipsing binary," and it gives astronomers a direct way to measure the mass and size of the stars.
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Researchers using NASA's Fermi Gamma-ray Space Telescope have discovered the
first gamma-ray pulsar in a galaxy other than our own. The object (
PSR J0540-6919) sets a new record for the most luminous gamma-ray pulsar known.
The pulsar with an age of roughly 1700 years lies in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a small galaxy that orbits our Milky Way and is located 163000 ly away.
The Tarantula Nebula is the largest, most active and most complex star-formation region in our galactic neighborhood. It's now clear that a
single pulsar, PSR J0540-6919 (or J0540), is responsible for roughly half of the gamma-ray brightness we originally thought came from the nebula.
The Tarantula Nebula was known to host two pulsars,
PSR J0540-6919 and PSR J0537-6910. J0540 spins just under 20 times a second, while J0537 whirls at nearly 62 times a second - the fastest-known rotation period for a young pulsar.
This campaign began as a search for a pulsar created by SN 1987A, the closest supernova seen since the invention of the telescope. That search failed, but it discovered J0537.
Prior to the launch of Fermi in 2008, only 7 gamma-ray pulsars were known.
To date, the mission has found more than 160.
