Hercules

Figure 3. Artist's impression of Her X-1. Credit: D. Klochkov.

Hercules (abreviation: Her), representing the mythical hero famed for his 12 labors, is a very large but unspectacular northern constellation. It contains the Keystone asterism, named after its shape, which is made by four of the main stars in Hercules – Epsilon, Zeta (Ruticulus), Eta, and Pi Her. See below for details of the constellation's brightest stars and interesting deep sky objects.

Hercules Cluster

Figure 2. The Hercules Cluster.

The Hercules Cluster (Abell 2151) is a loose unsymmetrical cluster of about 75 bright galaxies, some 500 million light-years away and about 6 million light-years across (Figure 2). It contains an unusually high proportion of spiral galaxies and also galaxies that shown evidence of interaction or other peculiar features (see galaxy interaction).

Hercules X-1

Hercules X-1 is an X-ray pulsar that is a member of an eclipsing binary system with an orbital period of 1.7 days (Figure 3); it lies about 15,000 light-years away. The visible component has been identified as the blue variable HZ Herculis, whose spectrum varies from late A or early F to B. Her X-l has a pulsation period of 1.2378 seconds, presumed to be its rotation period, and shows a 35-day quasi-periodicity in the X-ray region (but not in the optical). It is thought to move in a nearly circular orbit, to be accreting matter from HZ Her, and to have a mass of about 0.7 solar mass. The orbital period is stable, but the pulsation period is speeding up at a rate of about 1 part in 100,000 per year. The X-ray eclipse lasts 0.24 days.

Stars brighter than magnitude 4.0
star	vis mag	abs mag	spec type	distance (ly)	RA (h m s)	Dec (° ' ")
Beta (Kornephorus)	2.78	-0.50	G8IIIa	148	16 30 13	+21 29 22
Zeta (Rutilicus)	2.81	2.64	F9IV	35	16 41 17	+31 36 10
Alpha (Rasalgethi)	3.00	-0.02	M5Ib-II	382	17 14 39	+14 23 25
Delta (Sarin)	3.12	1.21	A3IV	79	17 15 02	+24 50 21
Pi	3.16	-2.10	K3IIab	367	17 15 03	+36 48 33
Mu	3.42	3.80	G5IV	27	17 46 27	+27 43 15
Nu	3.48	0.80	G8II	112	16 42 54	+38 55 20
Xi	3.70	0.20	G8III	163	17 57 46	+29 14 52
Gamma	3.70	0.61	K0III	135	16 21 55	+19 09 11
Iota	3.82	-2.09	B3V	495	17 39 28	+46 00 23
Omicron	3.84	-1.30	B9.5V	347	18 07 32	+28 45 45
109	3.85	0.87	K2.5IIIab	128	18 23 42	+21 46 11
Theta	3.86	-2.70	K1IIaCN	670	17 56 15	+37 15 02
Tau	3.91	-1.02	B5IV	315	16 19 44	+46 18 48
Epsilon	3.92	0.43	A0V	163	17 00 17	+30 55 35

Other objects of interest
name	type of object	notes
NGC 6210	planetary nebula	A bright inner ring surrounded by a faint outer ring. Magnitude 9.7; diameter 13 × 20" (inner), 20" × 43" (outer); RA 16h 44.5m, Dec +23° 49'
Great Cluster in Hercules	globular cluster	M13 (NGC 6205). See separate entry
M92 (NGC 6341)	globular cluster	NGC 5139. See separate entry