Birth and life cycle of stars
The stars that shine at night do not stay the same forever; they are born, grow old and finally change.
A nebula (तेजोमेघ) is a cloud mostly of hydrogen gas and dust. Gravity pulls it together until it becomes dense and round. When the pressure and temperature at the core rise, energy generation starts, and the hot sphere of gas is a star.
Rule 1: Life of an ordinary star — Nebula → ordinary star → red giant → planetary nebula → white dwarf. Our Sun is an ordinary star.
- Easy : The Sun is an ordinary star.
- Medium : An ordinary star ends as a white dwarf.
- Exam-type : Stars are born from nebulae, so a nebula is the "birthplace" of stars.
Rule 2: Life of a massive star — Nebula → massive star → red supergiant → supernova → neutron star or black hole. The mass of the star decides its last stage.
- Easy : The explosion of a massive star is a supernova.
- Medium : After a supernova a neutron star or a black hole forms.
- Exam-type : A Sun-like star does not become a supernova; it ends as a white dwarf.
Birth and life cycle of stars
In the app you can move this figure with your finger.
Open in the appOur Sun is a small part of the galaxy called the Milky Way, which has lakhs of stars.
Thinking stars never change. A star's life cycle takes millions to billions of years.
Memory clue : "Nebula → Star → Giant → End" — the end depends on mass: dwarf, neutron star or black hole.
Spot the difference
| Type | Stages | End |
|---|---|---|
| Ordinary star | Nebula, star, red giant | White dwarf |
| Massive star | Nebula, star, supergiant, supernova | Neutron star / black hole |
Practice questions
- 1What is a nebula?
- 2From what are stars born?
- 3What kind of star is our Sun?
- 4How does a nebula become dense and round?
- 5What is the last stage of an ordinary star?
- 6What is a supernova?
- 7On what does the final stage of a star depend?
- 8When does energy generation start as a nebula becomes a star?
Show answers (try them yourself first)
- 1A cloud of hydrogen gas and dust.
- 2From a nebula.
- 3An ordinary star.
- 4Gravity pulls its particles together.
- 5A white dwarf.
- 6The explosion of a massive star.
- 7On the mass of the star.
- 8When the pressure and temperature at the core become very high.
Terms used in sky watching
Standing in an open field and looking afar, the sky seems to touch the ground, and above us the sky looks like a dome.
The circle where sky and ground seem to meet is the horizon (क्षितिज). The imaginary sphere on which stars and planets seem to move is the celestial sphere (खगोल).
Rule 1: Six terms on the celestial sphere — (1) Zenith: the point exactly overhead. (2) Nadir: the point exactly below our feet. (3) Celestial poles: where the Earth's axis, extended, meets the celestial sphere. (4) Meridian: the great circle through both celestial poles, the zenith and the nadir. (5) Celestial equator: the circle made by extending the Earth's equator. (6) Ecliptic: the circle of the Sun's apparent yearly path.
- Easy : The point right overhead is the zenith.
- Medium : The point under our feet is the nadir, exactly opposite to the zenith.
- Exam-type : The Sun's apparent path is known as the ecliptic.
Rule 2: Stars rise earlier every day — Because the Earth rotates from west to east, the Sun, Moon and stars seem to rise in the east and set in the west. Stars rise 4 minutes earlier every day. Against the background of stars the Sun shifts about 1° eastwards a day and the Moon 12-13°.
- Easy : If a star rises at 7 pm today it rises at 6:56 pm tomorrow.
- Medium : After 15 days the star rises 15 × 4 = 60 minutes, that is 1 hour, earlier.
- Exam-type : A star that rises at 9 pm today will rise 2 hours earlier, at 7 pm, after 30 days.
Terms used in sky watching
In the app you can move this figure with your finger.
Open in the appDo sky watching away from the city, preferably on a new moon night; use binoculars; use the Pole Star as a reference.
Thinking stars rise at the same time every day. They rise about 4 minutes earlier each day.
Memory clue : "Day: 4 minutes; month: 2 hours" — 30 × 4 = 120 minutes.
Spot the difference
| Term | Meaning |
|---|---|
| Horizon | Circle where sky meets ground |
| Zenith | Point overhead |
| Nadir | Point below the feet |
| Ecliptic | Sun's apparent path |
Practice questions
- 1What is the horizon?
- 2What is the zenith?
- 3What is the nadir?
- 4What is the ecliptic?
- 5How many minutes earlier do stars rise each day?
- 6Why do the Sun, Moon and stars seem to rise in the east?
- 7A star rises at 8 pm today. When will it rise after 15 days?
- 8Why is a new moon night away from the city best for sky watching?
Show answers (try them yourself first)
- 1The circle where the sky and ground seem to meet.
- 2The point exactly overhead.
- 3The point exactly below our feet.
- 4The circle of the Sun's apparent yearly path.
- 5About 4 minutes.
- 6Because the Earth rotates from west to east.
- 71 hour earlier, at 7 pm.
- 8There is less artificial light and moonlight, so stars are seen clearly.
Constellations and the Pole Star
At night seven stars appear in the shape of a kite; with their help we can find the north.
A group of stars in a small part of the sky is a constellation (तारकासमूह). Some seem to form shapes of animals, humans or things. Western observers have divided the sky into 88 constellations.
Rule 1: Saptarshi and Sharmishtha show the Pole Star — Saptarshi (Great Bear) is a group of seven bright stars shaped like a kite: a quadrangle with a tail of three stars. Extending one side of the quadrangle leads to the Pole Star. Sharmishtha (Cassiopeia) is a group of five stars in the shape of an M; the perpendicular bisector of the line joining its third and fourth stars goes towards the Pole Star. Saptarshi is on one side of the Pole Star and Sharmishtha on the other.
- Easy : The Pole Star shows north.
- Medium : When Saptarshi sets, Sharmishtha rises, so the Pole Star can always be found.
- Exam-type : Once the Pole Star is found the north is known, which makes identifying other constellations easier.
Rule 2: Orion and Scorpio — Mrug (Orion) is easy to identify on winter nights: four stars form a quadrangle, and extending the line through the three middle stars reaches the very bright Vyadh (Sirius). In Vrushchik (Scorpio), a group of 10-12 stars, the brightest star is Jyeshtha (Antares).
- Easy : Vyadh is a very bright star.
- Medium : The line of the three middle stars of Orion points to Vyadh.
- Exam-type : The brightest star in Scorpio is Jyeshtha, also known as Antares.
Hold the sky map overhead. East is drawn on the right and west on the left; keep the direction you face at the bottom.
Thinking the Pole Star is the brightest star. It is not; it is important because it shows north.
Memory clue : "Saptarshi + Sharmishtha = two guards of the Pole Star" — when one sets, the other rises.
Spot the difference
| Constellation | Identification | Use |
|---|---|---|
| Saptarshi | 7 stars, kite shape | Finding the Pole Star |
| Sharmishtha | 5 stars, M shape | Finding the Pole Star |
| Mrug (Orion) | Quadrangle and three middle stars | Finding Vyadh |
| Vrushchik (Scorpio) | 10-12 stars | Identifying Jyeshtha |
Practice questions
- 1What is a constellation?
- 2How many stars are there in Saptarshi?
- 3Which letter shape does Sharmishtha look like?
- 4Why is the Pole Star important?
- 5How is Vyadh (Sirius) found?
- 6Which is the brightest star in Scorpio?
- 7What is the joint use of Saptarshi and Sharmishtha?
- 8Why is east shown on the right of a sky map?
Show answers (try them yourself first)
- 1A group of stars in a small part of the sky.
- 2Seven.
- 3M.
- 4It shows north and is a reference point for sky watching.
- 5By extending the line of the three middle stars of Orion.
- 6Jyeshtha (Antares).
- 7When one sets, the other rises, so the Pole Star can always be found.
- 8The map is held overhead, so the directions appear reversed.
Zodiac signs, nakshatras and the Sun's apparent path
On Makar Sankranti we say "the Sun entered Capricorn". In reality the Sun stays where it is; the Earth moves around it.
Dividing the ecliptic into 12 equal parts gives parts of 30° each; each part is a zodiac sign (रास). The part the Moon covers in one day is a nakshatra; dividing 360° into 27 equal parts gives about 13° 20′ each.
Rule 1: 12 signs and 27 nakshatras — 12 signs: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces. The Moon takes about 27.3 days for one round of the Earth, hence 27 nakshatras. The brightest star in a nakshatra is its yogatara. One sign holds 27 ÷ 12 = 2.25 nakshatras.
- Easy : 360° ÷ 12 = 30°, so one sign spans 30°.
- Medium : 360° ÷ 27 ≈ 13.3°, so one nakshatra spans about 13° 20′.
- Exam-type : 30° ÷ 13.33° ≈ 2.25, so one sign holds two and a quarter nakshatras.
Zodiac signs, nakshatras and the Sun's apparent path
In the app you can move this figure with your finger.
Open in the appRule 2: Apparent motion of the Sun — As the Earth moves around the Sun, the constellation seen behind the Sun keeps changing. This is the Sun's apparent motion, and the ecliptic is its apparent path. The Sun's rising in the east and setting in the west is also apparent motion. According to science, planets, stars and nakshatras do not influence human life.
- Easy : The Sun stays; the Earth moves, so the Sun seems to move.
- Medium : On Makar Sankranti the Capricorn constellation lies behind the Sun.
- Exam-type : "The Sun enters Mrug nakshatra in the rainy season" means the Mrug nakshatra lies behind the Sun then.
The study of signs and nakshatras is part of astronomy; it has no scientific link with predicting the future.
Thinking the Sun actually moves when it changes sign. It is the Earth's movement that makes it look so.
Memory clue : "12 signs × 30° = 360°; 27 nakshatras × 13° 20′ = 360°" — both make a full circle.
Spot the difference
| Point | Zodiac sign | Nakshatra |
|---|---|---|
| Number | 12 | 27 |
| Angle of each | 30° | about 13° 20′ |
| Based on | Parts of the ecliptic | Moon's path in one day |
Practice questions
- 1How many zodiac signs are there?
- 2How many nakshatras are there?
- 3What angle does one sign span?
- 4What is a yogatara?
- 5What is the apparent motion of the Sun?
- 6Why are there 27 nakshatras?
- 7How many nakshatras fit in one sign?
- 8What does "the Sun entered Capricorn" mean?
Show answers (try them yourself first)
- 112.
- 227.
- 330°.
- 4The brightest star in a nakshatra.
- 5The Sun seeming to move because of the Earth's motion.
- 6The Moon takes about 27.3 days to go once around the Earth.
- 727 ÷ 12 = 2.25.
- 8Because of the Earth's position, the Capricorn constellation lies behind the Sun.