OBSERVING THE NIGHT SKY

1
OBSERVING THE NIGHT SKY

• 1. Getting Started Stars and Constellations

2
What We Will Learn

• The nature of the night sky
• What changes and what doesnt
• Mapping the sky
• About stars and constellations

3
If you went out on a clear starry night and
looked up, what you might see may look like this
..
4
The Sky Looks Confusing...
5
There are thousands of stars. And many look
alike. How can we bring order to this confusion?
6
A useful way is to divide the sky into groups of
stars. A nice way to group them is such that
they form interesting patterns. These groups
are called Constellations. Now it is not so
confusing
7
Bringing Some Order to Chaos...
8
Mapping the night sky

• The stars are almost fixed in position relative
  to one another.
• This permits the sky to be mapped.
• Sky charts and maps can be used locate stars and
  other objects in the sky.

9
If India were the night sky, the towns would be
the stars, The constellations would be the
different states. As the different states help to
map our country, The constellations help to map
the night sky.
10
Before we learn how to use star charts and the
constellations to get familiar with the sky we
need to learn about how the night sky appears to
change over a night and over the months
and seasons
11
Changes in the night sky

• The night sky appears to change over a night
  this is due to the rotation of the earth around
  its axis once in 24 hours.
• The night sky at a given time appears to change
  over the months and seasons this is due to the
  revolution of the earth about the sun once in
  365.25 days.

12
Night and Day on Earth

In this picture the sun is to the left night is
to the right. So the stars can only be seen on
the right. Earth rotates once in 24 hrs from west
to east. So the stars appear to rise in the east
and set in the west.
13
Earths revolution around the sun- The ecliptic
23½
23

• Earths rotational axis is inclined at 66½ to
  the plane of the earths motion around the
  sun. This inclination causes the seasons.
• The plane of the earths motion around the sun
  is called the ecliptic.
• The ecliptic is also the plane of the apparent
  motion of the sun around the
• earth.

14
As Earth moves around the Sun, different stars
and constellations are seen. The stars that are
seen are the ones in the night sky, away from the
Sun. This explains why the night sky looks
different in the different seasons.
15
Objects which move or change

• The planets - move close to the ecliptic.
• Comets and shooting stars or meteors.
• Moons or planetary satellites.
• Variable stars, novae and supernovae
• Objects which move with respect to the fixed
• stars are not shown on star charts.
• We will discuss some of these in a later
  talk.

16
Stars

• Are hot, luminous gaseous bodies that generate
  heat and light through nuclear reactions.
• Many visible stars have been given names. We
  will discuss how stars are named.
• Stars belong to different magnitude classes,
• depending on their brightness.

17
Star names

• The brighter stars have 3 types of names
• Common or popular names, eg Pole Star
• Names in various languages, eg Polaris in
• Latin, or Dhruva in Indian languages.
• Names based on their positions in their
• constellations, eg ? (alpha) Ursae Minoris.

18
Star Brightness - Magnitude System

• Magnitude rank or importance
• 1st magnitude stars are the brightest stars.
• 6th magnitude stars, barely visible naked eye
• 5 magnitudes difference 100x brightness
• Each order of magnitude 2.512x brightness
• Thus 1st mag stars are 2.5 times as bright as
• 2nd mag stars, etc

19
The 10 Brightest Stars
20
The Constellations

• Groups of stars that often appear to form
  interesting patterns in the night sky.
• IAU-designated boundaries, marking out specific
  regions of the night sky. Useful
• in charting or mapping the sky.
• Names are in Latin, Sanskrit etc.
• The night sky is mapped by 88 constellations.
  

21
We will now look at some well known
constellations.
22
Orion the Hunter

• The most prominent Winter constellation is Orion
  the hunter.
• Some of its names in the Indian languages are
  Mahavyadha, Mrigarashi and Kalapurush.
• It is a very useful constellation to know because
  it can be used to point to other stars and
  constellations

23
The principal stars of the Orion constellation
24
Orion the Hunter A fanciful illustration
25
The Orion constellation as a region of the sky
26
Orion in the night sky
27
Ursa Major the Great Bear

• The most important constellation in the north sky
  is Ursa Major or Great Bear. An Indian name
  Saptarshi Mandala.
• Seven of its stars form the Big Dipper.
• Important because two stars in the Big Dipper
  point to the Pole Star.

28
The Great Northern Bear
29
The Seven Rishis or the Big Dipper
30
Ursa Major as a region of the sky
31
Leo the Lion

• Leo or Simharashi is a prominent constellation
  best seen in late Spring and early Summer.
• The principal stars of the constellaton seem to
  form a crouching lion.
• The stars in the head of the lion seem to from a
  sickle or koithi.

32
Principal stars of the constellation Leo
33
Leo as a standing lion
34
The Leo region of the night sky
35
Sagittarius Scorpius

• Finally, a most beautiful part of the night sky,
  the Sagittarius (Dhanurashi) Scorpius
  (Vrishchikarashi) region.
• This region teems with interesting objects
  nebulae and star clusters.
• The centre of our galaxy, the Milky Way,
• lies in this part of the sky.

36
The Sagittarius Scorpius region
37
Schematic of the Sagittarius Scorpius region
38
The scorpion
39
The boundaries of Scorpius
40
In summary, it is important to get to know well
the brightest stars and constellations. They
can then be used as pointers to the fainter
parts of the night sky.