The Sky In Autumn September October November

After the monsoon rains the autumn sky appears crystal clear, with little dust and smoke. Moreover, by September the Sun sets early, making longer periods of darkness available for starwatching. This is also the time when we can see the famous Andromeda Galaxy, the farthest object visible to the unaided sky.

Cygnus

To the east of Lyra is a beautiful constellation Cygnus, the Swan. One of the most prominent constellations of the northern sky, its stars are supposed to represent a flying swan which they do remarkably well. Its brightest star is Alpha Cygni, or Deneb (mag. 1.25) which forms the 'tail' of the swan. Deneb is one of the biggest giant stars known, being about 70,000 times brighter than the Sun and lies at a distance of 1825 light-years from us. The star Beta Cygni, or Albireo (mag. 3.0) forms the head of the swan. The stars Delta Cygni (mag. 2.87) and Epsilon Cygni (mag. 2.46) form the wing tips of the swan. Together the five main stars of the constellation form a distinct 'cross' with Deneb at its top. For this reason it is also known as the 'Northern Cross'. Deneb culminates at around 9.00 p.m. during the third week of September.

Albireo is the faintest of the five stars that make up the cross. But if we look through a pair of good binoculars

DRACO

CEPHEUS

HERCULES

LACERTA

Vega

CYGNUS

Cygnus

Star Name Magnitude Distance

(Light-years)

Star Name Magnitude Distance

(Light-years)

a

Deneb

1.25

1825

Q

Albireo

3.08

390

y

Sadr

2.20

750

5

2.87

160

e

Gienah

2.46

82

or a medium-power telescope we will find it to be a double star made up of a golden-yellow star with a fainter, bluish-green companion.lt is said that Albireo is the most impressive double star in the sky. We can find it out ourselves.

Another object worthy of attention is the star Chi Cygni which lies almost midway between the stars Beta Cygni and Gamma Cygni. It is what astronomers call a long-period variable. Its brightness changes from magnitude 4.5 to almost invisible magnitude 14 within a span of about 400 days. When it is in its brighter phase we can easily pick it out, but in its dimmer phase we may find it impossible to locate. So if we cannot see it now, we shouldn't lose heart. All we need to do is wait till it brightens up again.

Since the Milky Way passes right through Cygnus, the constellation is rich in star fields and clusters and contains several interesting objects. Immediately to the east of Deneb is the famous 'North America' nebula (NGC7000) which is so called because of its striking resemblance to the outline of that continent. The nebula is not visible to the unaided eye, but on a clear, moonless night, far away from city lights, we can pick it out with a pair of binoculars (10 x 50). Another interesting object is the open cluster M39 which lies northeast of Deneb. We can easily make out the individual stars with a pair of binoculars. Viewers in southern parts of India will see both Lyra and Cygnus over the northern horizon although from north India both can be seen almost overhead at the time of culmination.

The Swan.

If we turn our gaze south of Cygnus, we will come to a brilliant white star Alpha Aqulae, or Altair (Shravana) in the constellation of Aquila, the Eagle. The constellation is supposed to look like the side view of an eagle with Altair (mag. 0.80) as one of its glittering eyes. We can easily identify Altair because it is flanked on two sides by fainter stars, like guarding sentinels. Along with Vega and Deneb, Altair forms what astronomers call the 'Summer Triangle' although the three are best visible in autumn. Shravana is one of the 27 nakshatras, and the Indian month of Shravan owes its name to Shravana as during this month the Moon attains fullness when seen in its vicinity. Altair culminates at around 9.00 p.m. during the first week of September.

To the southwest of Altair and just west of the star Lambda Aquilae (mag. 3.44) is a beautiful star cluster Mil. A group of about 200 stars visible only through a pair of binoculars or a telescope, it is also known as the 'Wild Duck' cluster. It gets its popular name because its stars are arranged in a fan-shape, like a flight of wild ducks, with a

SAGITTA I B

HERCULES

DELPHINUS

Altair

AGUILA

AQUARIUS

brighter orange star at the apex. We can see the individual stars more clearly if we use a powerful telescope. Mil is

Aquila

Star

Name

Magnitude

Distance (Light-years)

a

Altair

0.77

17

P

Alshain

3.71

36

Y

Tarazed

2.72

284

X

Althalimain

3.44

98

C

Dheneb

2.99

5

3.36

52

part of a tiny constellation called Scutum, the Shield, which has no stars brighter than magnitude 5.

part of a tiny constellation called Scutum, the Shield, which has no stars brighter than magnitude 5.

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