UPCAT REVIEWER: GENERAL SCIENCE (ASTRONOMY)

Universe

• the vast gaseous space where galaxies, stars, planets, moons, and other heavenly bodies suspended and move with perfect orderliness.

Big-bang Theory

• First proposed by Georges Lemaître.

• He suggested that the universe began from a very hot and dense point, often called the primeval atom, later known as YLEM.

Steady state theory

• the universe has no beginning and no end.

• As the universe expands and galaxies move apart, new matter is continuously created to fill the empty spaces.

• This keeps the universe looking the same over time.

Oscillating universe theory

• suggests that the universe goes through a cycle: it expands (like in the Big Bang), then slows down, contracts due to gravity (called the "Big Crunch"), and then possibly expands again — repeating this process over and over.

Solar system

• composed of the Sun and all Bodies that revolve around it.

Mercury
Venus
Earth
Mars

Four solid planets in the solar system:

Jupiter
Saturn
Uranus
Neptune

Four Gas planets in the solar system: (J S U N)

Ptolemaic system
Copernican system

The models of Universe:

Ptolemaic system

• Proposed by Claudius Ptolemy

• The Earth is the center of the universe (geocentric)

Copernican system

• Proposed by Nicolaus Copernicus

• The Sun is the center of the solar system (heliocentric).

Law of Ellipses
Law of Equal areas
Law of Harmonies

Kepler’s Laws of Planetary Motion:

Law of Ellipses

• Planets orbit the sun in elliptical paths, not perfect circles.

Law of Equal areas

• A planet moves faster when closer to the sun, and slower when farther.

Law of Harmonies

• The square of a planet's orbital period (T²) is proportional to the cube of its average distance from the sun (R³).

Planetesimal Theory
Companion Star Theory (Binary star hypothesis)
Nebular hypothesis

Theories of the Origin of the Solar System:

Planetesimal theory

• Proposed by Thomas Chamberlin and Forest Moulton

• States that a passing star came near the Sun, and the gravitational pull drew out gas and dust from the Sun.

• These materials cooled and solidified into small bodies called _, which later formed the planets.

Companion Star Theory (Binary star hypothesis)

• Suggests that the Sun once had a _ in a binary system.

• This star either exploded or broke apart, and the resulting materials formed the solar system.

Nebular hypothesis

• Proposed by Immanuel Kant and Pierre-Simon Laplace

• Suggests that the solar system formed from a large rotating cloud of gas and dust (a nebula).

• This _ collapsed under gravity, flattened into a disk (not a saucer), and started to spin.

• The Sun formed at the center, and the planets formed from the remaining materials.

Moon

• it is the earth’s natural satellite, its distance from earth is 384,000 km.

• ¼ size of the earth.

Solar eclipse

• happens when the Moon passes between the Sun and the Earth, blocking the Sun's light either partially or totally.

• happens during new moon phase.

Lunar eclipse

• occurs when the Earth comes between the Sun and the Moon, and the Earth’s shadow falls on the Moon, making it look dark or reddish.

• happens during a full moon phase.

Umbra

• The darkest part of the shadow where the light source is completely blocked.

• total eclipse

Penumbra

• The lighter outer part of the shadow where the light source is only partially blocked.

• Partial eclipse.

Revolution

• the movement of the Earth around the Sun along its elliptical orbit.

• it takes about 365.25 days to complete one revolution, which causes the seasons.

Rotation

• the spinning of the Earth on its own axis.

• it takes about 24 hours and causes day and night.

23.5 degrees

• the tilt of the Earth's axis relative to the vertical (or perpendicular to the plane of its orbit).

• this tilt is responsible for the changing seasons.

Galaxies

• massive systems composed of billions of stars, planets, gas, and dust, all held together by gravity.

• An example is the_, where our solar system is located.

Photosphere
Sunspot
Chromosphere
Corona
Reversing

Atmospheric Layers of the Sun: (P S C C R)

Photosphere

• the visible surface of the Sun; this is the light we see.

Sunspot

• dark, cooler areas on the photosphere caused by magnetic activity.

Chromosphere

• a layer above the photosphere; it appears reddish or pinkish during a solar eclipse due to the hydrogen gas.

Corona

• the outermost part of the Sun’s atmosphere; it appears as a white halo during a total solar eclipse.

Reversing

• a thin layer above the photosphere that absorbs certain wavelengths, producing dark lines (absorption lines) in the solar spectrum.

Stars

• mostly composed of 72% hydrogen, 25% helium and 3% other materials (carbon, iron, nickel)

Neutron star

• A _ is an extremely dense object made almost entirely of neutrons.

• It forms when a massive star explodes in a supernova and the core collapses under its own gravity.

Black Hole

• A _ is a region in space where gravity is so strong that nothing can escape, not even light.

• It forms when a very massive star collapses completely after a supernova.

• The boundary around it is called the event horizon — once something crosses it, it cannot escape.

Asteroid

• A small, rocky, irregularly shaped object that orbits the Sun.

• Mostly found in the asteroid belt between Mars and Jupiter.

Comet

• A small celestial body made of ice, dust, and rock.

• When it gets close to the Sun, heat vaporizes the ice, creating a glowing coma and a tail that always points away from the Sun due to solar wind.

Meteor

• A meteoroid that enters Earth’s atmosphere and burns up due to friction, creating a streak of light (commonly called a "shooting star").

Meteorite

• A fragment of a meteoroid that survives its journey through the atmosphere and lands on Earth.

Meteoroids

• A small chunk of rock or metal in space, often from an asteroid or comet.

• It becomes a meteor when it enters Earth’s atmosphere, and a meteorite if it lands on Earth.