Monoceros
What is Monoceros?
Constellations· Winter evenings (N) · centre of the Winter Triangle. Monoceros, the unicorn, lies in the Milky Way inside the Winter Triangle. Its stars are faint, but it holds some of winter's finest nebulae and star clusters, such as the Rosette Nebula and the Christmas Tree Cluster.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Beta Monocerotis | 3.7 |
| Alpha Monocerotis | 3.9 |
The 2 brightest of the 3 stars used in the figure (2 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | winter evenings (N) |
|---|---|
| Brightest star | Beta Monocerotis (3.7, triple) |
| How to find | inside the Betelgeuse–Sirius–Procyon triangle |
| Area rank | 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
V838 Monocerotis, the star that suddenly swelled in 2002
In January 2002 an unremarkable star in Monoceros suddenly brightened. At its peak it was about a million times as bright as the Sun. It was named V838 Monocerotis. But it did not behave like an ordinary exploding star. After brightening, its surface cooled and turned red, and the star swelled to more than 1,000 times the Sun's diameter.
Why this happened is still not fully settled. The leading idea among astronomers is that two stars orbiting close together merged into one. The merger released a huge amount of energy at once, and that made the star puff up. V838 Monocerotis is about 19,000 light-years from Earth.
What made the event famous came afterwards. The star had long been wrapped in clouds of dust. As the flash of light spread outward, it lit up this dust layer by layer, and the Hubble Space Telescope photographed the scene over several years. The dust itself was not moving; what moved was the place the light had reached. This is called a light echo.
The constellation in depth
Monoceros does not come from Greek myth. The region between Orion, Canis Major and Canis Minor has no bright stars, so for a long time it had no name. Around 1612 the Dutch mapmaker Petrus Plancius filled the gap with the unicorn of legend. The name itself is Greek for "one horn".
In this constellation the objects between the stars are more famous than the stars. Because it lies in the Milky Way, it is full of gas clouds and young star clusters. The best known is the Rosette Nebula, a cloud of gas more than 100 light-years across. The young cluster NGC 2244 at its centre lights up the gas, so in photographs it looks like a red rose.
The Christmas Tree Cluster, NGC 2264, gets its name from the tree-like pattern of its stars. At its tip sits the Cone Nebula, a pointed cone of dark dust. Hubble's Variable Nebula, NGC 2261, changes the shape of its bright parts within weeks. Astronomers explain that clumps of dust passing close to the star at its tip cast moving shadows on the cloud.
On evenings from December to February, Betelgeuse, Sirius and Procyon form a large triangle. Almost everything inside this Winter Triangle belongs to Monoceros. Its stars are only about 4th magnitude, so they are hard to connect from a city, but the triangle makes the area easy to find. Its brightest star, Beta, splits into three stars in a small telescope.
Frequently asked
- When can I see Monoceros?
- Monoceros is best seen in winter evenings (N).
- How do I find Monoceros?
- To find Monoceros, look for inside the Betelgeuse–Sirius–Procyon triangle.
- What is the brightest star in Monoceros?
- The brightest star in Monoceros is Beta Monocerotis, at magnitude 3.7.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.