Musca
What is Musca?
Constellations· Southern autumn · right below the Southern Cross. A small Fly clinging just below the Southern Cross. Set in the Milky Way, it is thick with stars, and part of the Coalsack, a dark cloud of dust, reaches into it.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Alpha Muscae | 2.7 |
| Beta Muscae | 3.0 |
The 2 brightest of the 2 stars used in the figure (1 lines). Lower magnitude means brighter.
Constellation basics
| Season | Apr–May evenings; never visible from London or New York |
|---|---|
| Brightest star | α Muscae, mag 2.7 |
| How to find | a small knot of stars just south of Crux |
| Area rank | 77th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The 'new star' of 1991 in Musca turned out to be a black hole
In January 1991 an object in Musca suddenly brightened. It was named Nova Muscae 1991; 'nova' means 'new star'. Unlike an ordinary nova, though, it was far brighter in X-rays than in visible light, so astronomers suspected something different was going on.
Closer study gave the answer. A star lighter than the Sun was circling an unseen object every 10.4 hours. From the companion's motion, the hidden object's mass came out at about seven times the Sun's. No neutron star can be that heavy, so the object was judged to be a black hole.
Most of the time this black hole is quiet. Gas pulled from the companion builds up slowly in a disc around it, and at some point the gas pours inward all at once and the system flares. The 1991 outburst was one of those moments; since then only the companion's faint light has been seen. Black holes like this reveal themselves only when they wake up.
The constellation in depth
Musca is one of twelve southern constellations that came from Dutch navigators' observations in the 1590s. It was first drawn as a bee and only later became a fly. Old charts often showed the neighbouring Chamaeleon sticking out its tongue to catch it. Because it sits right below the Southern Cross, it is easy to find.
The brightest star, Alpha Muscae, is a blue-white star of magnitude 2.7. It belongs to the Scorpius–Centaurus association, a group of hot stars born at about the same time. Its brightness also changes very slightly and regularly, because the star gently swells and shrinks.
Lying in the heart of the Milky Way, Musca has plenty to see. The Coalsack beside the Southern Cross is a dark cloud of dust that blocks the starlight behind it, and its southern part reaches into Musca. The Hourglass Nebula, MyCn 18, is here too; a planetary nebula like this is the shell of gas thrown off by a dying star. The globular clusters NGC 4833 and NGC 4372, balls of old stars, show in a small telescope.
At declinations of −64° to −75° it cannot be seen from Europe or most of the United States. From the southern hemisphere, look on April and May evenings, when the Southern Cross is high, for the small group of stars just below it. In the Sky view, set Sydney or Cape Town on an April evening and the Fly appears just below the Cross.
Frequently asked
- When can I see Musca?
- Musca is best seen in Apr–May evenings; never visible from London or New York.
- How do I find Musca?
- To find Musca, look for a small knot of stars just south of Crux.
- What is the brightest star in Musca?
- The brightest star in Musca is Alpha Muscae, at magnitude 2.7.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.