Aries - Constellation Guide

Aries

Arietis

Ari

The Ram



Introduction

Aries is a medium-sized northern constellation of the zodiac that lies in a rather barren part of the sky. It contains mostly inconspicuous faint stars and is bordered by Taurus to the east, Perseus and Triangulum to the north, Pisces to west and Cetus to the south. The constellation was one of the original 48 constellations plotted by second century astronomer Ptolemy and remains today as one of the modern 88 constellations defined by the IAU (International Astronomical Union).

In Greek mythology, Aries represents the golden ram that was sent to rescue Phrixos and Helle, the children of King Athamus of Boeotia and his first wife Nephele. The King's second wife, Ino, was resentful and wanted the children, in particular, Phrixos killed. She induced a famine in the Kingdom and then falsified a message to the King, indicating Phrixos must be sacrificed in order to save the land. Athamus was about to sacrifice his son, when Aries - sent by Nephele - arrived. The ram managed to rescue Phrixos, but Helle didn't survive. Phrixus then sacrificed the ram to Zeus with its Golden Fleece presented to King Aeëtes of Colchis. In a later myth, Jason and the Argonauts who actively sought the fleece eventually managed to steal it. In ancient Egyptian astronomy, Aries was associated with the god Amon-Ra, who was depicted as a man with a ram's head and represented fertility and creativity. The Arabs knew Aries as a sheep and the Chinese as a dog.

The constellation contains a single second magnitude star, one third magnitude star, three fourth magnitude stars and over a dozen fifth magnitude stars. It contains no Messier objects and no bright deep sky objects. However, its boundaries contain numerous faint galaxies within the range of medium / large size amateur scopes. For small scopes, there are a handful of nice double stars, including some bright ones.

Two thousand or so years ago Aries contained the vernal equinox, the point where the Sun annually passes from south to north across the celestial equator. This occurs on or about March 20th and signals the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Due to precession the vernal equinox has now moved into neighbouring Pisces.

Aries covers 441 square degrees of sky, ranking it 39th in overall size. It's best seen from Northern Hemisphere locations during the months of October, November and December. There are several meteor showers that radiate from Aries, including the Daytime Arietids and the Delta Arietids.

Aries Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Brightest Stars

Hamal (alpha Arietis - α Ari) – at mag. +2.01 is the brightest star in Aries. It's a K2 orange giant about twice as massive as the Sun that's located 66 light-years distant. The star is variable although the range is extremely small (mag. +1.98 to +2.04). From 2,000 BC to 100 BC, Hamal was positioned at the vernal equinox.

Sheratan (beta Arietis - β Ari) – is a spectroscopic binary system located 59.6 light-years from Earth. The primary star is a white A5V type main sequence star that completes a highly elliptical orbit with its companion every 107 days. The secondary star is intrinsically fainter and although its spectral type has not been accurately determined it could be F type or even G type similar to the Sun. The stars shine with a combined magnitude of +2.64.

Double Stars

Mesartim (gamma Arietis - γ Ari) – is a striking double star consisting of equally bright (mag. +4.5) white-blue components. With a separation of 7.6 arc seconds the two stars are resolvable with any size scope and are widely considered to be one of the prettiest equal pairs. Their combined magnitude is +3.9 and they lie about 164 light-years away.

Mesartim was one of the first double stars to be telescopically identified. English scientist Robert Hooke accidentally stumbled across it, while searching for a comet in 1664.

Epsilon Arietis (ε Ari) – is a challenging double of two almost equally bright white stars. Together they shine at magnitude +4.6 but are separated by only 1.4 arc seconds. Telescopes of 100mm (4-inch) aperture should split them, but nights of steady transparency are required. A magnification of 200x or so should suffice.

The Epsilon Arietis system is located 330 light-years distant. Both stars are classified as A2 V with the primary at magnitude +5.2 and the secondary magnitude +5.6. When first identified as a double by F.G.W. Struve in 1827 the two stars were separated by just 0.5 arc seconds.

Lambda Arietis (λ Ari) – mag. +4.8 is located 2 degrees west of Hamal. This wide separated double (40 arc seconds) is easily split with small telescopes. The primary star is white (mag. +4.9) and the fainter secondary yellow (mag. +7.7). The pair can be resolved with 7x50 binoculars, although the task is much easier with 10x50 models due to the increased magnification.

Lambda Arietis is located 129 light-years distant.

Pi Arietis (π Ari) – is a multiple star system about 800 light-years distant. The main star is a magnitude +5.3 blue giant star that's separated by 3.3 arc seconds from its magnitude +8.5 companion. For small telescopes owners, this is a challenging split due to the faintness of the secondary and the closeness of the pair. A good quality 100mm (4-inch) refractor on good nights should do the trick, but it needs to be pushed towards the higher end of its magnification range.

A third fainter star with a separation 25.2 arc seconds (mag. +10.8) is visible in larger scopes. In addition, the primary itself is a spectroscopic binary making this a 4 star system.

30 Arietis – a 6th magnitude wide double for small scopes. Both stars are white in colour (mag. +6.5 and +7.0). With a separation of 38 arc seconds they are resolvable in any size scope. The primary component is also a spectroscopic double itself and therefore this is a triple star system.

33 Arietis – consists of a primary pure white star of magnitude +5.4 with a magnitude +8.4 companion, separated by 24 arc seconds. With a combined magnitude of +5.3, it's just about visible to the naked eye. A small scope 80mm (3.1-inch) scope at about 50x magnification will easily split the pair.

41 Arietis – mag. +3.6, is the brightest star that makes up a small faint naked eye triangle along with 35 Arietis (mag. +4.7) and 39 Arietis (mag. +4.5). This grouping formed the basis of the short-lived and now obsolete constellation, known as the Musca Borealis, or the Northern Fly. The fly that hovered over the back of Aries, was finally swatted for good at the beginning of the 20th century when the modern 88 constellations were defined.

A number of faint stars surround 41 Arietis making it a multiple star, but all bar one are due to line of sight effects. The only true companion is separated from the main star by just 0.2 arc seconds and can't be seen visually, but can be detected by spectroscopic methods. The fainter surrounding stars visible through medium sized amateur scopes are of magnitudes +8.8 (sep. 121 arc secs), +10.6 (sep. 27 arc secs) and +11.0 (sep. 33 arc secs).

Variable Stars

U Arietis – is a Mira type variable star that varies between magnitudes +7.2 and +15.2 over a period of 371 days. At its brightest, it's easily visible with 7x50 or 10x50 binoculars. When faintest, a minimum 300mm (14-inch) scope is recommended to spot it.

U Arietis is located 1,630 light-years distant.

R Arietis – is another Mira type variable star that's 4,100 light-years away. It varies between magnitudes +7.4 and +13.7 over 187 days. When at its brightest, and like U Arietis, the star is easily visible with 7x50 or 10x50 binoculars.

SX Arietis (56 Arietis) – is an unusual star worth mentioning. It varies in brightness by only 0.1 magnitudes over a period of about a day. What makes it interesting is that it's a rotating helium variable star and the prototype of its class. About 50 such stars in this class, have so far been identified.

With an apparent magnitude of +5.8, SX Arietis is faintly visible to the naked eye, but the brightness variations are far too small to be noticed visually.

Deep Sky

Galaxies

NGC 772 – is the largest and brightest of the galaxies in Aries. It's a magnitude +10.3 type SA(s)b unbarred spiral galaxy visible in small scopes. In total, it spans 7.2 x 4.3 arc minutes of apparent sky. Located 105 million light-years from us, NGC 772 is an extremely large galaxy with an actual diameter of 220,000 light-years. For comparison, this is twice the size of our Milky Way and 1.5 times the size of the Andromeda Galaxy (M31), the largest Local Group member. NGC 772 is estimated to contain at least 1 trillion stars.

Locating NGC 772 is quite easy. The galaxy is positioned 2 degrees east-southeast of Mesartim and forms a small right-angle triangle with Mesartim and Sheratan. A 100mm (4-inch) telescope at low powers reveals a faint oval misty patch of light, spread over 3 arc minutes. On nights of good seeing, the nucleus appears well defined and surrounded by a hazy nebulosity, representing the spiral arms. It's possible to glimpse the spiral arms using a 200mm (8-inch) scope, but they are much easier to see in larger backyard scopes of 300mm (12-inch) aperture or more.

As a result of gravitational tidal forces, probably from nearby satellite galaxy NGC 770, one of the spiral arms in NGC 772 is more developed that the others. It appears longer and slightly elongated and as a result Halton Arp included NGC 772, as Arp 78, in his Atlas of Peculiar Galaxies.

To date, two supernovae SN 2003hl and SN 2003iq have been observed in NGC 772.

NGC 770 – observers with very large backyard scopes maybe able to spot NGC 770 in the same field of view as NGC 772. This is the largest satellite galaxy of NGC 772, but it's faint at magnitude +14.1 and spans only 1.1 x 0.8 arc minutes in diameter. A 400mm (16-inch) scope reveals an object that looks like an out of focus star. At high magnifications, it's possible to notice a small amount of detail including the nucleus.

NGC 821 – is a mag. +10.8 type E6 elliptical galaxy located in the far southern section of Aries close to the border with Cetus and Pisces. It's positioned 2.5 degrees north of star ξ1 Cet (mag. +4.3) and can be spotted with 100mm (4-inch) scopes, appearing as a small faint round nebulous patch of light. NGC 821 has an apparent size of 2.5 x 1.7 arc minutes. When viewed through larger backyard scopes, the surface appears uneven in brightness with some mottling visible.

The galaxy is 80 million light-years distant with an actual diameter of 60,000 light-years.

NGC 972 – is an inclined spiral galaxy at the northern section of the constellation. It's positioned 3.5 degrees northwest of double star 41 Arietis (mag. +3.6). Located just off an imaginary line connecting NGC 972 and 41 Arietis is star 35 Arietis (mag. +4.7).

At magnitude +11.4, NGC 972 is best seen with at least a medium size scope. Through a 200mm (8-inch) instrument it appears as an elongated misty envelope of nebulosity with a brighter centre, just north of a faint double star. The galaxy covers 3.3 x 1.7 arc minutes, but has a reasonably high surface brightness.

NGC 1156 – is another galaxy located near to star 41 Arietis, but this time in the opposite direction to NGC 972. It's an irregular object, positioned 3 degrees southeast of the star. At magnitude +11.7, you would probably not expect much from this galaxy but be in for a pleasant surprise. Even small scopes show some details, including a faint ghostly like halo surrounding a brighter core with a number of foreground stars also visible in the same field of view. On nights of good seeing, a 200mm (8-inch) scope reveals mottling with larger instruments displaying subtle texturing in the halo.

NGC 877 – mag. +11.8 is a spiral galaxy 1.5 degrees southeast of 19 Arietis (mag. +5.7). There are two 8th magnitude stars adjacent to NGC 877 and the galaxy appears in the same low power field of view as one of these stars. A 200mm (8-inch) scope shows little detail, except for a slightly brighter core. Increasing the aperture does not significantly enhance the view, although the galaxy appears larger and hints at structural details. It's located 180 million light-years distant.

NGC 678 and NGC 680 – are a pair of 12th magnitude galaxies, very close together in the eastern part of the constellation near to the Pisces border. They are separated by only 5 arc minutes, which physically corresponds to about 200,000 light-years. NGC 678 and 680 are currently interacting with each other, resulting in shape distortions. They are part of the NGC 691 group of galaxies and are both located 125 million light-years distant.

Of the two, NGC 680 is an elliptical galaxy and the brighter at magnitude +11.9. It measures 1.8 x 1.6 arc minutes. Although larger in both apparent and absolute size, NGC 678 is not as bright as NGC 680. This spiral galaxy shines at magnitude +12.5 with an apparent size of 4.5 x 0.8 arc minutes. It appears almost edge on from our perspective.

Both galaxies are visible in medium sized telescopes, but are fairly faint. What's noticeable are the different shapes; NGC 680 is essential round, NGC 678 needle like. A prominent dust lane bisects through NGC 678 and it can be seen in larger amateur scopes.

NGC 697 – is a barred spiral galaxy with an apparent magnitude of +12.0 that's located 2 degrees northwest of Sharatan. A 200mm (8-inch) scope reveals a highly inclined thin elongated patch of light. It stands out quite well due to a reasonably high surface brightness. There is a 12th magnitude star positioned at the eastern edge of NGC 697.

The galaxy NGC 674 is widely believed to be a mistake entry in the NGC catalogue and is though to be the same object as NGC 697.

Meteor showers

Aries is home to a number of meteor showers, including the Daytime Arietids. This is one of the strongest showers that occurs during the day and lasts from May 22nd to July 2nd. At peak on June 7th, about 50 meteors per hour can be detected using radar techniques. However, it's possible to visually spot a few meteors before dawn towards the eastern horizon.

Of the night-time Arietids, all have low peak rates of the order of a few meteors per hour. This includes the Delta Arietids in December but it can produce some bright fireballs.

Aries Star Data Table

Henry Draper Catalogue (HD) Hipparcos Catalogue (HIP) Bayer Flamsteed Struve Name RA (J2000) DEC (J2000) Visual Mag. Var. Var. Mag. Range Period (days) Double Sep. (arc secs) PA (deg.) Mag. Primary. Sec 12929 9884 Alpha Arietis 13 --- Hamal 02h 07m 10s 23d 27m 45s 2.01 Y 2.19 -> 1.98 uncertain --- --- --- --- 11636 8903 Beta Arietis 6 --- Sheratan 01h 54m 38s 20d 48m 29s 2.64 --- --- --- --- --- --- --- 11502 8832 Gamma Arietis 5 180 Mesartim 01h 53m 32s 19d 17m 38s 3.88 --- --- --- Y 7.6 1 4.52 / 4.58 18519 13914 Epsilon Arietis 48 333 Epsilon Arietis 02h 59m 13s 21d 20m 26s 4.63 --- --- --- Y 1.4 210 5.17 / 5.57 11973 9153 Lambda Arietis 9 --- Lambda Arietis 01h 57m 56s 23d 35m 46s 4.79 --- --- --- Y 40 44 4.80 / 7.70 17543 13165 Pi Arietis 42 311 Pi Arietis 02h 49m 18s 17d 27m 52s 5.26 --- --- --- Y 3.3 121 5.32 / 8.50 16246 12189 30 Arietis 30 4005 30 Arietis 02h 37m 01s 24d 38m 50s 6.48 --- --- --- Y 38 275 6.50 / 7.02 16628 12489 33 Arietis 33 289 33 Arietis 02h 40m 41s 27d 03m 39s 5.30 --- --- --- Y 23.6 356 5.30 / 8.40 17573 13209 41 Arietis 41 --- 41 Arietis 02h 49m 59s 27d 15m 38s 3.61 --- --- --- Y 0.2 / 121 / 27 / 33 290 / 233 / 230 / 290 3.63 / 8.80 / 10.66 / 11.04 19737 --- U Arietis --- --- U Arietis 03h 11m 03s 14d 48m 00s 7.20 Y 15.2 -> 7.2 371 --- --- --- --- 13913 10576 R Arietis --- --- R Arietis 02h 16m 07s 25d 03m 07s 7.40 Y 14.3 -> 7.4 187 --- --- --- --- 19832 14893 56 Arietis 56 --- SX Arietis 03h 12m 14s 27d 15m 25s 5.78 Y 5.78 -> 5.74 0.73 --- --- --- ---

Aries Deep Sky Data Table