Stock 8 and IC 417 in Auriga
December 2024 - Nebula and Cluster of the Month
As it draws to a close, we experience the year’s longest and coldest nights. Riding high is the celestial charioteer, Auriga. Historical images of Auriga show the charioteer in his chariot, holding the reins with his right hand and with a goat and two kids under his left arm. The identity of the charioteer is in some dispute, and several semi-mythical (or wholly mythical) individuals have been claimed as the charioteer. There are many, quite diverse, stories about how he came to be placed in the heavens. Strangely, none of these include a goat, and nobody seems to know where that came from.
The constellation straddles the galactic equator and is rich in open clusters, three of which are in Charles Messier’s list – M36, M37 and M38. The NGC adds another 14 open clusters to the tally, and the IC a further three. That’s not the final word, though. There are many catalogues of open clusters and the Basel, Berkeley, Collinder, Czernik, Dolidze, King, Lund and Stock open cluster catalogues (no doubt amongst others) all contain objects within Auriga. Some of these are relistings of others. For example, all the Collinder clusters in Auriga are also NGC objects. Three of them are the Messier objects. It all gets a little confusing.
Stock 8 is an open cluster with (as far as I can tell) four aliases. In addition to its Stock listing, it is also Lund 178 and bears the modern designations of OCl 436 and the ungainly C0524+343. Stock 8 it is, then.
Stock 8 is located just to the east of the 5th magnitude star φ Aurigae, which lies near the middle of the southern half of the constellation’s ‘kite’ asterism.
Stock 8 is associated with the emission nebula IC 417. The cluster is very young, with measured ages all giving an upper limit of five million years. IC 417 is the stellar nursery where the cluster was born. A photometric study1 of the cluster, published in The Astrophysical Journal in 2019, revealed 51 short-period variables that were found to be members of the cluster. Of these 53, 28 were Main Sequence stars of β Cephei- and δ Scuti-type. Variable stars of β Cep-type are usually young, B-type stars with small variations caused by pulsations. δ Sct-type stars (also sometimes known as ‘dwarf Cepheids’ are also pulsating stars, still young but with spectral types typically in the range A0-F5. The remaining 23 variables were Pre-Main Sequence T Tauri-type stars. These are stars in the late-formation phase, still working their way towards the Main Sequence of the Hertzsprung-Russel diagram. Typically, they have masses of less than three solar masses. The Sun probably spent some time early in its career as a T Tauri star.
Streams of material within IC 417 have been found to contain many Young Stellar Objects (YSOs) that are not associated with Stock 8, showing that IC 417 is still an active star-forming region and may be in the process of birthing another open cluster.
As usual, there are slight differences in authors’ estimates of the cluster’s Trumpler type. Uranometria (as far as I can tell from their somewhat verbose expansions of Trumpler types) rates it I2m n, with which the Night Sky Field Guide agrees, whereas Archinal & Hynes rate it I3m n. The only difference here is the range of magnitudes ascribed to the member stars. Both describe it as well detached with a strong central condensation, fairly rich in stars and associated with nebulosity.
The cluster is around 5’ across and contains 40 stars or thereabouts, the brightest of which is magnitude 9.0.
To observe this young open cluster, centre your telescope field on the fifth-magnitude φ Aurigae. The cluster is located about 6’ slightly north of east of it. Note that in atlases, it is nearly always placed in the wrong location, 4’ north of φ Aurigae. This positional inaccuracy is a not-uncommon problem with some of the more obscure open cluster catalogues, and one that the observer should always be aware of. In this case, the cluster is located 5.6’ E and 2.6’ N of the usually-given location. For those with Goto capabilities, use the RA and Declination given in the table below.
The cluster may not be immediately obvious. Although the brightest star is magnitude 9, the magnitudes of the other members stretch down into the mid-teens, so you may not see all 40 stars. Nevertheless, the impression is of a fairly rich, compact cluster. The brighter stars shine against a backdrop of fainter sparkles. On exceptionally good nights, a hint of the emission nebula IC 417 can be seen as a faint haziness around the cluster. On my observation shown here, Stock 8 is the clustering of stars east of the brightest star in the field (which is φ Aurigae).
May I take this opportunity to wish you all best wishes for Christmas or whatever Midwinter festival you choose to observe.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| Stock 8 | 05h 28m 04s | +34° 27’ 06” | Open cluster | |
| IC 417 | 05h 28m 13s | +34° 25’ 10” | Emission nebula |
References:
- Sneh Latah et al, Short-period Variable Stars in the Young Open Cluster Stock 8, 2019 AJ 158 68
