The document serves as an introduction to astronomy, explaining that it encompasses the study of everything beyond Earth's atmosphere, including celestial bodies and mysterious phenomena like dark matter and dark energy, which together constitute the majority of the universe. It highlights the ways astronomers observe the universe—through telescopes like Hubble and James Webb, and space missions—and emphasizes the importance of understanding our place in the cosmos. Lastly, it describes the astronomical units of measurement used for defining vast distances in space, such as astronomical units, light years, and parsecs.

1. Introduction to Astronomy
PHYS 006
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Chapter 1
Astronomy and the Universe

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Clear, dark, moonless night
What do you see and what do you think of?

3. THOUGHTS ?
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What are the light dots in the sky?
What is the Sun? the Earth? the moon?
Where did the Earth, Moon, and Sun come from?
What are the planets and stars made of?
How did the universe begin?

4. Wondering about the universe is a key part of what makes us human.
Our curiosity, our desire to explore and discover, and most important,
our ability to reason about what we have discovered are qualities that
distinguish us from other creatures.
The study of the stars transcends all boundaries of culture, geography,
and politics.
Astronomy is a universal subject – its subject is the entire universe.
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What is Astronomy?
Astronomy:
Astron = star
Nomos = law
Astronomy is the study of everything in the universe beyond Earth’s atmosphere, including:
꙳ objects we can see with our naked eyes, like the Sun , the Moon , the planets, and the stars
꙳ objects we can only see with telescopes or other instruments, like faraway galaxies and tiny particles.
꙳ questions about things we can't see at all, like dark matter and dark energy .
Astronomy = Law of the stars

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What is Dark Matter?
Do you know that there are things in the universe that we can’t detect, no matter
what instruments we use?
They’re called dark matter and dark energy.
꙳ Dark matter is a hypothesized form of matter particle that does not emit
electromagnetic radiation.
꙳ In other words, Dark matter doesn't give off light like a galaxy or absorb
light like a black hole.
꙳ The existence of dark matter is inferred from gravitational effects on
visible matter, such as stars and galaxies.

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What is Dark Energy?
Same as dark matter, dark energy has been one of the most mysterious issues it
exists in science.
꙳ Dark Energy, in physical cosmology and astronomy, it is an unknown form of
energy which is imagined to fill all of space, tending to accelerate the
expansion of the universe.
꙳ Dark energy produces an effect opposite to the force of gravity, thus opposing
the approach and subsequent collision of all the elements that make up the
cosmos (universe), and by contrast it is responsible for the continued expansion
of the universe.
꙳ Dark energy is the most accepted hypothesis to explain the observations since
the 1990s indicating that the universe is expanding at an accelerating rate.
꙳ It turns out that roughly 75% of the Universe is dark energy. Dark matter
makes up about 21%. The rest - everything on Earth, everything ever
observed with all of our instruments, all normal matter - adds up to less than
4% of the Universe.
꙳ Scientists still don’t know very much about dark matter and dark energy, but
they are hard at work trying to find out more.
Maybe one of those scientists will be you!
Dark
matter
75%

8. 1. By exploring the planets, astronomers uncover clues about the formation of the solar
system
8
Why to Study Astronomy?

9. 2. By studying stars and nebulae, astronomers discover how stars are born, grow old and
die
A nebula is a giant cloud of dust and gas in space, which the stars are born in.
9
Why to Study Astronomy?
Butterfly nebula
by The Hubble Space Telescope

10. 3. By observing galaxies, astronomers learn about the origin and fate of the universe
A Galaxy: is a large system of stars, gases, interstellar dust and dark matter held
together by gravitation and isolated from similar systems by vast regions of space.
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Why to Study Astronomy?
This spectacular galaxy,
called M63, contains
about a hundred billion
stars.

11. 4. To know our place in the cosmos
Milky way galaxy  solar system  Earth
11
Why to Study Astronomy?

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We are made of star-stuff.
“The cosmos is within us.
We are a way for the universe
to know itself.” - Carl Sagan

13. How do we observe the universe?
1. Local Telescopes
2. Space Telescopes, examples:
• Hubble Space Telescope:
Because it orbits outside the Earth’s atmosphere in the vacuum
of space, the Hubble Space Telescope (HST) can detect not only
visible light but also ultraviolet and near-infrared light coming
from distant stars and galaxies. These forms of nonvisible light
are absorbed by our atmosphere and hence are difficult or
impossible to detect with a telescope on the Earth’s surface.
• James Webb Space Telescope:
The James Webb Space Telescope (JWST) is a space telescope
designed primarily to conduct infrared astronomy. As the largest
optical telescope in space, its greatly improved infrared
resolution and sensitivity allow it to view objects too old, distant,
or faint for the Hubble Space Telescope. 13
Tools in Studying Astronomy?
Hubble Space Telescope
James Webb Space Telescope

14. How do we observe the universe?
3. The International Space Station (ISS):
The International Space Station is a large spacecraft. It orbits around Earth.
It is a home where astronauts live. The space station is also a science lab.
4. Space Shuttle Orbiters:
An orbiter is a space plane that goes into orbit around the earth. Examples:
Columbia, Discovery, and Endeavour
5. Space expeditions:
An expedition to the ISS refers to the crew that is occupying the space
station and using it for research and testing.
6. Automated spacecraft:
Automated spacecraft for solar system exploration come in many shapes
and sizes. While they are designed to fulfill separate and specific mission
objectives. Examples:
• New horizon is first spacecraft to explore Pluto.
• Deep space is originally designed to test a dozen new technologies
and flew by an asteroid and a comet. 14
Tools in Studying Astronomy?
The International Space Station
Space Shuttle Endeavour in orbit

15. How do we observe the universe?
7. Robots (NASA has sent five robotic vehicles, called rovers, to Mars.)
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Tools in Studying Astronomy?

16. Astronomical Distances
The sky is vast and distances between objects can be very large
Astronomers define new units. They are, in order of increasing length:
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Tools in Studying Astronomy?
Astronomical Unit (AU)
Light year (ly)
Parsec (pc)

17. Astronomical Distances
17
Tools in Studying Astronomy?
Astronomical Unit (AU)
When discussing distances across the solar system, astronomers
use a unit of length called the astronomical unit (abbreviated AU).
It is the average distance from the Earth to the Sun
1 AU = 150 million km = 150 ×106 km = 1.5 × 1011 m
It is used to express the average distance between:
• the Sun and other planets
• the planets in the solar system

18. Astronomical Distances
18
Tools in Studying Astronomy?
To talk about distances to the stars, astronomers use two different
units of length.
The light-year (abbreviated ly) is the distance that light travels in
vacuum in a time of one year.
𝑑 = 𝑣 × 𝑡
1 𝑙𝑦 = 3 × 108 𝑚
𝑠𝑒𝑐
× 365
𝑑𝑎𝑦𝑠
𝑦𝑒𝑎𝑟
× 24
ℎ
𝑑𝑎𝑦
× 60
𝑚𝑖𝑛
ℎ
× 60
𝑠𝑒𝑐
𝑚𝑖𝑛
1 𝑙𝑦 = 9.46 × 1015𝑚
Light year (ly)

19. Astronomical Distances
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Tools in Studying Astronomy?
To talk about distances to the stars, astronomers use two different
units of length.
The parsec (abbreviated pc) is the distance at which a radius of 1 AU
subtends an angle of 1 arcsecond.
𝟏 𝒂𝒓𝒄𝒔𝒆𝒄𝒐𝒏𝒅 =
𝟏
𝟑𝟔𝟎𝟎
°
tan 1 𝑎𝑟𝑐𝑠𝑒𝑐𝑜𝑛𝑑 =
𝑜𝑝𝑝𝑜𝑠𝑖𝑡𝑒
𝑎𝑑𝑗𝑎𝑐𝑒𝑛𝑡
=
1 𝐴𝑈
1 𝑝𝑐
1 𝑝𝑐 =
1 𝐴𝑈
tan 1 𝑎𝑟𝑐𝑠𝑒𝑐𝑜𝑛𝑑
=
1.5 × 1011 𝑚
tan
1
3600
°
Parsec (pc)
𝟏 𝒑𝒄 = 𝟑. 𝟏 × 𝟏𝟎𝟏𝟔 𝒎
𝟏 𝒑𝒄 = 𝟑. 𝟐𝟔 𝒍𝒚

20. Angular Distances
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Tools in Studying Astronomy?
Astronomers use angles to denote the positions and
apparent sizes of objects in the sky.
Angular measure describes the size of an angle exactly.
The basic unit of angular measure is the degree, designated
by the symbol °.
A full circle is divided into 360°, and a right angle measures 90°.
It is used to describe:
• the angular distance between two stars
• the apparent size of a celestial object—that is, what fraction
of the sky that object seems to cover.
Example: the angular diameter (or angular size) of
the Moon is ½ °.

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The adult human hand extended to arm’s
length can be used to estimate angular
distances and angular sizes in the sky.
Estimating Angles with Your Hand
Tools in Studying Astronomy?

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