This chapter takes us on a journey to explore the objects in the night sky, beyond our Earth. It begins with the experience of children in Ladakh, where the clear, dark skies allow for stunning views of stars, sparking curiosity about the vastness of space.

Stars And Constellations

What Are Stars And Constellations?

When we look at the night sky, we see countless bright dots. These are stars, which are celestial bodies that shine with their own light. Some stars appear brighter or dimmer than others, depending on their actual brightness and distance from Earth.

Groups of stars sometimes appear to form recognizable patterns in the sky. Historically, people imagined these patterns resembling familiar objects, animals, or characters from stories. These groups of stars forming patterns were traditionally called constellations.

More formally, constellations are now defined as specific regions of the sky that include these star patterns. There are 88 officially recognized constellations that divide the entire sky.

Identifying patterns among stars is not just for fun; it has been a crucial skill for understanding the night sky.

Historical Use Of Constellations

In ancient times, long before modern technology like GPS or even magnetic compasses, people used stars and constellations for navigation. Recognizing specific star patterns helped sailors and travelers find directions on land or at sea, especially at night.

Different cultures developed their own names and stories for these patterns, making it easier for them to remember and locate stars in the sky.

Prominent Constellations

Some constellations are particularly famous and relatively easy to spot in the night sky (depending on location and time of year).

• Orion: Often depicted as a hunter. It features three bright stars in a line that form the hunter's belt. Orion is typically visible during winter months in the Northern Hemisphere. Nearby constellations include Canis Major (containing the brightest star in the night sky, Sirius) and Taurus (the Bull).
• The Big Dipper: A prominent pattern of seven bright stars, shaped like a ladle or dipper. It is part of the larger constellation Ursa Major (the Great Bear). In India, the Big Dipper is known as Saptaṛiṣhi.
• The Little Dipper: A smaller pattern of seven stars, also ladle-shaped, part of the constellation Ursa Minor (the Little Bear).
• The Pole Star (Polaris): Located at the end of the handle of the Little Dipper. The Pole Star is significant because it appears almost stationary in the northern part of the sky. It helps to easily locate the North direction in the Northern Hemisphere. In India, it is known as Dhruva tārā.

Extra Information: Indian astronomy has traditional names for stars and groups of stars, called `nakṣhatra`, which sometimes align with stars in Western constellations (e.g., Ārdrā is Betelgeuse in Orion, Rohiṇī is Aldebaran in Taurus, Kṛittikā are the Pleiades in Taurus).

Night Sky Watching

Visibility Of The Night Sky

The clarity of the night sky depends on several factors:

• Clouds: A clear, cloudless night is necessary to see stars.
• Moonlight: A moonless night is best for viewing fainter stars and constellations, as bright moonlight can make them harder to see.
• Light Pollution: Artificial lights from cities scatter light in the atmosphere, making the sky brighter and reducing the visibility of stars. Dark locations away from city lights offer the best views.
• Air Pollution (Smoke and Dust): Pollutants in the air also reduce visibility.
• Obstructions: Tall buildings or trees can block the view of parts of the sky.

Observing from open, dark areas significantly enhances the night sky viewing experience.

Extra Information: Light pollution is a growing global problem. Dark sky reserves and parks are established to preserve areas with minimal light pollution for astronomical research and public viewing. The Hanle Dark Sky Reserve in Ladakh, India, is an example of such an initiative.

Preparation For Viewing

To have a successful night sky watching session:

• Choose a suitable location: A dark, open area away from lights and obstructions. Always go with an adult for safety.
• Plan the timing: Choose a moonless, clear night. Consider what stars or constellations are visible from your location at that specific time of year and night.
• Allow eyes to adjust: After arriving at the dark location, wait for about 30 minutes for your eyes to adapt to the darkness. This improves your ability to see fainter objects.

Tools For Observation

Tools that can be helpful:

• Sky Maps/Apps: Mobile phone apps (like Sky Map, Stellarium) or printed star charts can help identify stars and constellations based on your location and time.
• Magnetic Compass: Useful for orienting yourself and finding cardinal directions (North, South, East, West).
• Notebook: For recording observations or drawing patterns.
• Binoculars or Telescope: For viewing objects brighter, larger, and seeing dimmer objects not visible with the naked eye.

Locating Constellations And Stars

You can locate specific constellations and stars by using their patterns relative to each other and their position in the sky at a given time.

• Locating the Pole Star: Find the Big Dipper. The two stars at the end of its "cup" are called the pointer stars. Imagine a straight line passing through these stars towards the North. The Pole Star is located along this line, at a distance about five times the distance between the two pointer stars. Once found, verify it by observing that all other stars appear to rotate around this relatively fixed star.
• Locating Sirius: First, find the constellation Orion (especially the three bright stars of the belt). Imagine a line extending downwards and towards the East through these three belt stars. This line points towards Sirius, the brightest star in the night sky.

Visibility of constellations changes with the season and time of night because of Earth's movement around the Sun and its rotation.

Our Solar System

Beyond the stars that form constellations, there are other important celestial objects, including the ones that are part of our cosmic neighbourhood.

Introduction To Our Solar System

Our Solar System is the system of celestial objects that are bound together by the gravity of the Sun. It consists of:

• The Sun (our star)
• Eight planets that revolve around the Sun
• The moons (natural satellites) that revolve around planets
• Many smaller objects, including asteroids and comets

Most objects in the Solar System are in motion, primarily revolving (orbiting) around the Sun. Planets also rotate on their own axis.

The Sun

The Sun is the star at the center of our Solar System. It is a massive, extremely hot ball of gases that generates enormous amounts of energy, emitted as heat and light. The Sun is the largest and heaviest object in the Solar System and is responsible for providing almost all the energy within the system.

Our Sun appears much larger and brighter than other stars because it is vastly closer to Earth compared to other stars.

The distance between the Earth and the Sun is approximately 150 million kilometers. This distance is defined as one astronomical unit (au), which is a convenient unit for measuring distances within the solar system.

Other stars are so far away (e.g., Proxima Centauri, the next closest star, is about 269,000 au away) that they appear only as points of light, and are not visible during the day because of the Sun's brightness.

Planets

A planet is a large, nearly spherical celestial body that revolves around the Sun and has cleared its orbital path of other debris. Currently, there are eight planets in our Solar System.

In order of increasing distance from the Sun, the eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

• Inner Planets: Mercury, Venus, Earth, and Mars are closer to the Sun. They are relatively smaller and have solid, rocky surfaces.
• Venus: Very bright, often called the Morning Star or Evening Star.
• Earth: Our home planet. Appears blue from space due to large water coverage, sometimes called the Blue Planet. Takes about one year to revolve around the Sun and 24 hours to rotate on its axis (defining a day).
• Mars: Appears reddish and is known as the Red Planet due to iron oxide in its soil.
• Outer Planets: Jupiter, Saturn, Uranus, and Neptune are farther from the Sun. They are much larger than the inner planets and are mainly composed of gases, often referred to as gas giants. These planets have large ring systems around them.

Planets get heat and light from the Sun. Generally, planets farther from the Sun are colder, although factors like atmospheric composition (e.g., Venus's thick atmosphere trapping heat) can affect their temperature.

Planets can be distinguished from stars in the night sky because planets generally do not twinkle like stars do. Planets are also seen at different positions in the sky over time as they orbit the Sun.

The brightest planet, Venus, is easily visible to the naked eye, often seen at dawn or dusk. Mercury, Mars, Jupiter, and Saturn can also be seen without a telescope, appearing as bright, non-twinkling points of light.

Extra Information: Pluto was previously considered the ninth planet but was reclassified as a dwarf planet in 2006 along with other similar small objects found in the outer Solar System.

Astronomical instruments like binoculars and telescopes allow us to see planets and other celestial objects more clearly and reveal dimmer objects not visible to the naked eye.

Natural Satellites (Moons)

Objects that revolve around planets are called satellites. Natural satellites are often referred to as moons.

The Moon is Earth's natural satellite. It revolves around the Earth in approximately 27 days. The Moon is Earth's nearest neighbor in space.

The Moon is about one-quarter the diameter of the Earth. Its surface is marked by circular bowl-shaped depressions called craters, formed by impacts from asteroids or rocks. The Moon has almost no atmosphere, water, or life.

Other planets also have moons. Mars has two moons, while gas giants like Jupiter and Saturn have many moons.

Extra Information: India has successfully launched missions to the Moon (Chandrayaan program), including the soft landing of Chandrayaan-3 near the Moon's south pole in 2023. August 23 is celebrated as 'National Space Day' in India to commemorate this achievement.

In a general sense, any object revolving around a much larger object can be called a satellite; for example, the Earth is a satellite of the Sun.

Asteroids

Asteroids are rocky, irregularly shaped objects that revolve around the Sun. They are much smaller than planets.

Many asteroids are found in a region between the orbits of Mars and Jupiter, called the asteroid belt. Occasionally, some asteroids pass close to Earth.

Comets

Comets are icy, rocky bodies that also revolve around the Sun, typically in highly elliptical orbits. They are sometimes called 'visitors from the outer Solar System'.

As a comet approaches the Sun, the ice and frozen gases heat up and evaporate, forming a bright cloud around the nucleus (coma) and a long, luminous tail of gas and dust that points away from the Sun.

Comets appear bright only when they are relatively close to the Sun. Many comets orbit the Sun periodically (like Halley's Comet, which appears every 76 years), while some may escape the Solar System or collide with other celestial bodies.

Extra Information: Historically, comets were sometimes viewed with fear or superstition, believed to bring bad luck, but we now understand them as natural celestial objects.

The Milky Way Galaxy

Our Home Galaxy: The Milky Way

When viewed from a very dark location on a moonless night, one can sometimes see a faint, extended band of light stretching across the sky. This band is part of our own galaxy, called the Milky Way Galaxy.

A galaxy is a massive system containing millions to billions of stars, gas, dust, and dark matter, all held together by gravity. Our Solar System, with the Sun and its planets, is just a tiny part of the vast Milky Way Galaxy.

The Milky Way is our galactic home.

The Universe

Beyond The Milky Way

The Universe is everything that exists, including all matter and energy, all space and time. It is vast beyond imagination.

Our Milky Way Galaxy is only one of countless galaxies in the Universe. There are many other galaxies beyond our own, separated by immense distances.

The Search For Life

Scientists are exploring the Universe to understand its structure, formation, and evolution. One of the big questions is whether life exists anywhere else besides Earth.

Much of this search focuses on exoplanets – planets discovered orbiting other stars in our galaxy. While many exoplanets have been found, scientists have not yet found definitive evidence of life on any of them. The search continues as we explore further into the Universe.

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