Читать книгу The Star Book: Stargazing throughout the seasons in the Northern Hemisphere - Peter Grego - Страница 6

Northern circumpolar constellations

From northern temperate climes a number of large constellations — most of them easily identifiable – remain constantly above the horizon throughout the year as they wheel anticlockwise (counterclockwise) around the north celestial pole.

Polaris, the tip of Ursa Minor’s tail, is conveniently located just one degree from the north celestial pole. The star is easily found by tracing a line from Merak and Dubhe in Ursa Major, two stars known as the Pointers. These two stars are part of the famous Plough, alternatively known as the Big Dipper or sometimes the Saucepan. One of the sky’s most easily identifiable asterisms, the Plough forms the tail and hindquarters of Ursa Major. At midnight during the autumn (fall) the Plough is at its lowest, scraping along the northern horizon, while it soars high overhead at springtime. On the other side of the north celestial pole can be found another prominent asterism, the W of Cassiopeia. The Plough and the W take turns throughout the year in gaining the high celestial ground.

Much of the relatively star-sparse region between the Pointers and the W is filled by the large but ill-defined constellation of Camelopardalis, while another large, faint constellation, Lynx, borders Camelopardalis and Ursa Major. Somewhat easier to trace is the House asterism of Cepheus, which lies between Cassiopeia and the pole. Between Cepheus and the tail of Ursa Major, skirting around the edge of Ursa Minor, is the sprawling constellation of Draco, covering an area of more than a thousand square degrees. Despite being so large, Draco contains only a few reasonably bright stars ranging between the second and third magnitude.

By way of contrast, on the other side of the celestial pole and nestled snugly between Cassiopeia and Auriga, lies beautiful Perseus. For the most part a circumpolar constellation from northern temperate regions, Perseus has a number of bright stars sprinkled along a section of the Milky Way and is one of the loveliest areas to scan with binoculars.

A broad view of the northern circumpolar sky, looking due north (east at right, west at left). The outer circle represents extent of circumpolarity from London (52°N) and the inner circle for stars that are circumpolar from New York (41°N). The ecliptic is also shown at either side (none of it is circumpolar). The chart is relevant for 1 November (4am), 1 December (2am), 1 January (midnight), 1 February (10pm) and 1 March (8pm).

Ursa Minor

UMi / Ursae Minoris

Highest at midnight: early January

Sometimes called the Little Dipper, Ursa Minor is a small but significant constellation incorporating the north celestial pole. Its brightest star, Polaris, is located within just one degree of the pole. A phenomenon called precession, caused by the slow movement of the Earth’s axis, will bring Polaris closest to the pole (within half a degree) at the end of the 21st century.

Polaris is useful to aim at when aligning an equatorial telescope for a quick observing session. A telescope roughly aligned with Polaris will keep an object within the field of view of a medium-power eyepiece for a long time before requiring adjustment. A telescope trained on Polaris itself will reveal it to be a double star, with an eighth-magnitude companion.

Beta UMi (Kochab) and Gamma UMi (Pherkad), the end stars of the Little Dipper, are sometimes called the Guardians of the Pole. A keen naked eye will discern a faint star close to Pherkad (about half the Moon’s diameter away) – this is an unrelated foreground star.

Cepheus

Cep / Cephei

Highest at midnight: late February

Cepheus contains several reasonably bright stars. Among the stellar delights of Cepheus is Beta Cep, a double star of magnitudes 3.2 and 7.9 that can be resolved through a small telescope. Beta Cep is a variable star, a blue giant whose brightness fluctuates by around one tenth of a magnitude over a period of just a few hours. Another variable, which is also a nice double, is Delta Cep. The prototype of the Cepheid variables. This is a pulsating yellow supergiant that varies between magnitudes 3.5 and 4.4 in a period of five days nine hours. Delta’s companion is a magnitude 6.3 blue star, and the pair is easy to separate through a small telescope.

Mu Cep is a red supergiant whose striking colour has earned it the nickname of the Garnet Star. Binoculars will show its ruddy hue to good effect. The star is also variable, fluctuating between magnitudes 3.4 and 5.1 over a period of two to two and a half years. Mu Cep is one of the biggest stars visible with the unaided eye – if placed in the position of the Sun, the surface of its enormous bloated sphere would extend almost out to the orbit of Saturn.

Xi Cep is a double star comprising a magnitude 4.4 blue star and a magnitude 6.5 orange giant, easily resolvable through a small telescope.

T Cep, a red Mira-type variable star, has a period of more than a year. At its maximum brightness the star just pops into the range of the unaided eye, shining at magnitude 5.2.

An extension of the Milky Way nudges into the southern part of Cepheus, and a couple of lovely open star clusters can be found in this vicinity – IC 1396 and NGC 7160 – both delightful to view through a 150mm telescope. IC 1396 is embedded within a sizeable patch of nebulosity; it is also known as the Elephant’s Trunk Nebula because of a prominent dark sinuous dust lane, which is visible on photographs. Large binoculars will reveal IC 1396 as a misty patch. NGC 7160 is a small, compact star cluster; around 30 of its stars are visible through a 200mm telescope, half a dozen of the brighter ones standing out from the rest.

The Garnet Star in Cepheus, observed through a 100mm refractor by the author.

Double star Delta Cephei.

The Elephant’s Trunk Nebula in Cepheus, imaged through an 80mm refractor with an astronomical CCD camera (filters used).

Ursa Major

UMa / Ursae Majoris

Highest at midnight: early March

Seven of the brightest stars within Ursa Major make up an asterism variously called the Plough or the Big Dipper. While this asterism itself doesn’t much look like a bear, a little time spent in tracing the traditional outline comprising the remainder of the constellation’s bright stars will convince any stargazer that the ancients who named it had an extremely good eye for form.

The two front stars of the Plough, Alpha UMa (Dubhe) and Beta UMa (Merak) are known as the Pointers, since an imaginary line extending from them leads to Polaris and the north celestial pole. Zeta UMa (Mizar), the second star of the Plough’s handle, has a fainter magnitude 4 partner, 80 UMa (Alcor), which is visible with the unaided eye. Mizar itself is a close double star, with components of magnitudes 2.2 and 3.8, separable with a small telescope.

A pair of galaxies bright enough to be seen through binoculars, Bode’s Galaxy (M81) and the Cigar Galaxy (M82) lie in the far north. Just half a degree apart, the pair is visible in the same low-power telescopic field. While M82 is almost edge-on to us, M81 is tilted at less of an angle. Some ten million light years distant, these galaxies are interacting with each other. On the other side of the constellation, the face-on spiral galaxy M101 is visible through binoculars as a circular smudge, and appears mottled through a 200mm telescope.

The Owl Nebula (M97), a faint planetary nebula, appears as a pale disk about twice the diameter of Jupiter through a 150mm telescope. The dark eyes of the owl, so obvious in many images, are rather elusive and require at least a 250mm telescope to discern. M108, a bright, sizeable and nearly edge-on galaxy, can fit into the same low-power telescopic field as M97. Bright condensations within M108 can be discerned through a 150mm telescope.

Although the location of the galaxy M109 can be identified fairly easily, being a little more than half a degree east of Gamma UMa, its low surface brightness makes it more of a challenge to observe. A 200mm telescope will reveal its bright elliptical centre along with a superimposed foreground star just to the north of the core, but detail within the spiral arms requires a larger instrument to resolve.

The familiar stars of the Plough in Ursa Major.

Multiple star Mizar in Ursa Major.

Galaxy M101 in Ursa Major, imaged using a 127mm refractor and astronomical CCD camera.

Planetary Nebula M97 in Ursa Major, imaged using a 127mm refractor and astronomical CCD camera.

Draco

Dra / Draconis

Highest at midnight: early July

Despite sprawling across a huge portion of the northern circumpolar region, Draco is not the most prominent of constellations. Its traditional outline can be traced from its head (marked by Beta Dra and Gamma Dra) just north of Hercules, along a winding path to Alpha Dra (Thuban) at the constellation’s narrowest part, around to Lambda Dra near Draco’s western border. Around the time that the Pyramids were constructed, Thuban (magnitude 3.7) was the brightest star near the north celestial pole; precession will grant it this distinction again in more than 21,000 years’ time.

Mu Dra is a close telescopic double star with white components of magnitudes 4.9 and 5.6; the pair is slowly moving apart, and they can now be comfortably resolved with a 100mm telescope, and are a good test for a 60mm telescope. Psi Dra is much easier to resolve; binoculars will reveal the yellow stellar duo of magnitudes 4.6 and 5.8. Binoculars can split the two wide components of 39 Dra (magnitudes 5 and 7.4); a telescope will show the magnitude 8 companion of the brighter star.

The Cat’s Eye Nebula (NGC 6543) is a small but bright planetary nebula with a distinct bluish hue. A 150mm telescope will show it as a small ring surrounding an eleventh-magnitude central star.

The Cat’s Eye Nebula in Draco, imaged using a 105mm refractor and astronomical CCD camera.

Cassiopeia

Cas / Cassiopeiae

Highest at midnight: early October

With its prominent five-star W asterism, Cassiopeia is one of the easiest constellations to recognize. Gamma Cas, the central star of the W asterism, is an irregular variable star that fluctuates, at unpredictable intervals, between magnitudes 3 and 1.6. Eta Cas is a nice double star with a magnitude 3.5 yellow primary and a red magnitude 7.5 companion, easily visible through a small telescope.

Cassiopeia is a joy to scan with binoculars, as a bright section of the Milky Way flows across the constellation, engulfing the W. A treasure trove of bright open clusters lies within its boundaries, most of which lie east of the W. Containing around 30 stars, the bright compact cluster of M103 is best seen at higher magnifications. The Owl Cluster (NGC 457) is a loose assembly of around 100 fairly bright stars arranged in distinct lines; its two brightest stars shine like an owl’s eyes. NGC 663 is a beautiful binocular cluster containing around 80 stars. On the far western side of Cassiopeia, the compact Scorpion Cluster (M52) contains around 100 stars, the brightest of which form a splendid S shape. The Bubble Nebula (NGC 7635), a faint diffuse nebula visible through a 200mm telescope, lies just half a degree southwest of M52, so that the two objects can be viewed in the same low-power field of view.

A delightful alignment of stars known as Kemble’s Cascade can be found in the circumpolar constellation of Camelopardalis, east of Cassiopeia.

The Bubble Nebula in Cassiopeia, imaged using an 80mm refractor and astronomical CCD camera (filters used).

Perseus

Per / Persei

Highest at midnight: mid-November

Crossed in the north by the Milky Way, Perseus is a magnificent constellation containing a number of bright stars and open clusters. Near Alpha Per (Mirfak) lies Melotte 20, a large loose star cluster made up of a snaking chain of bright stars; it is a stunning sight through binoculars and at low magnifications.

Beta Per (Algol) is a famous eclipsing binary. Every 2.87 days it drops from magnitude 2.1 to 3.4, changes easily monitored with the unaided eye. The Spiral Cluster (M34) can just be discerned with the unaided eye some five degrees west of Algol. It contains a number of star chains, with some of its brighter stars paired up.

Eta Per is a nicely coloured double star, easily resolvable through a small telescope, with an orange magnitude 3.8 primary and a blue magnitude 8.5 companion.