Garnet Star
| Polish version is here |
The star known as the Garnet Star (µ Cep) is an impressive red supergiant located in the constellation Cepheus, right on the edge of the open-cluster and nebula IC 1396. It is among the brightest naked-eye stars in our Galaxy. William Herschel gave the star its name after he first noted its striking red hue, likening it to the gemstone garnet.
Early in its life, the star’s mass may have been about 20 solar masses, which hastened its evolution. It is now in an advanced, late stage, nearing its end. Like other massive red supergiants—such as η Carinae and WR 102ka—it could soon conclude its life as a supernova. The core left behind would collapse into a neutron star or a black hole.
Observations
May 27 2025, around 11:00 PM – Katowice, Poland
Conditions: urban sky, heavy light pollution
The Garnet Star (Mu Cephei) is a red supergiant of spectral type M2 Ia and a semi-regular variable star. It is one of the largest supergiants visible to the naked eye—if placed at the Sun’s position, its outer layers would extend beyond Jupiter’s orbit. Mu Cephei is about 350,000 times more brighter than the Sun.
There’s no denying that the star is a spectacular sight, and Herschel wasn’t exaggerating when he gave it a name reminiscent of a beautiful gemstone (Photo 1).
The Garnet Star’s apparent brightness varies between 3.45m and 5.1m, a range readily discernible to the naked eye. Two overlapping periods have been identified: a shorter one of about 730 days and a longer one of roughly 4,400 days. These pulsations result from cyclic absorption and release of energy within the star, causing its outer layers to alternately expand and contract. Violent internal processes eject large amounts of material, forming concentric shells of dust and gas. A surrounding layer of water vapor has also been detected, making µ Cep a compelling target for infrared studies.
Photo 1 Parameters:
- Total exposure time: 10 minutes (stack of 40 RAW frames at 15s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 2000
- Newton telescope (150/750), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Further readings:
- Perryman, M. A. C., The HIPPARCOS Catalogue, Astronomy and Astrophysics, 323, 1997, L49–L52
- N. Tetzlaff, R. Neuhäuser, M. M. Hohle, A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun, Monthly Notices of the Royal Astronomical Society, 410 (1), 2011, pp. 190–200
- Emily M. Levesque, Philip Massey, K. A. G. Olsen, Bertrand Plez, The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought, The Astrophysical Journal, 628 (2), 2005, pp. 973–985
Marek Ples