Observing the Universe

This lesson covers how astronomers observe and measure the universe — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand the advantages of space telescopes over ground-based telescopes, how absorption spectra reveal what stars are made of, and key units of astronomical distance.

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Telescopes — Our Windows to the Universe

Telescopes are instruments that collect and focus electromagnetic radiation from distant objects in space. There are two main types based on their location:

Ground-Based Telescopes

These are telescopes located on Earth's surface. They are typically placed on high mountains in dry, remote locations to minimise atmospheric interference.

Feature	Detail
Location	On Earth's surface (ideally high altitude, dry climate)
Types	Optical (visible light), radio telescopes
Advantages	Cheaper to build and maintain; easier to repair and upgrade; can be very large
Disadvantages	Affected by atmospheric absorption and distortion; limited by weather, light pollution, and the day-night cycle
Famous examples	Very Large Telescope (Chile), Jodrell Bank (UK), Arecibo (Puerto Rico, now collapsed)

Space Telescopes

These are telescopes placed in orbit around the Earth (or at other positions in space).

Feature	Detail
Location	In orbit around Earth or at a Lagrange point
Types	Optical, infrared, ultraviolet, X-ray, gamma-ray
Advantages	No atmospheric absorption or distortion; can observe 24/7; can detect wavelengths absorbed by Earth's atmosphere
Disadvantages	Very expensive to build and launch; difficult and costly to maintain or repair
Famous examples	Hubble Space Telescope (launched 1990), James Webb Space Telescope (launched 2021)

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Advantages of Space Telescopes

Space telescopes have several important advantages over ground-based telescopes:

1. No Atmospheric Absorption

The Earth's atmosphere absorbs many wavelengths of electromagnetic radiation, including most ultraviolet, X-rays, gamma rays and parts of the infrared spectrum. Only visible light and radio waves pass through the atmosphere relatively well.

A space telescope is above the atmosphere and can therefore detect all wavelengths — including those that never reach the ground.

Wavelength	Reaches Ground?	Detectable from Space?
Radio waves	Yes	Yes
Microwaves	Partially	Yes
Infrared	Partially (some absorbed by water vapour)	Yes
Visible light	Yes	Yes
Ultraviolet	Mostly absorbed by ozone	Yes
X-rays	Absorbed by atmosphere	Yes
Gamma rays	Absorbed by atmosphere	Yes

2. No Atmospheric Distortion

The atmosphere causes stars to twinkle (scintillation) — the air is turbulent and constantly moving, which distorts the light passing through it. This limits the sharpness (resolution) of images from ground-based telescopes.

Space telescopes are above the atmosphere, so they produce much sharper, clearer images.

3. No Weather or Light Pollution

• Ground-based telescopes cannot observe when it is cloudy or when there is light pollution from nearby cities.
• Space telescopes can observe continuously, 24 hours a day, without interruption from weather or daylight.

4. Can Observe in All Directions

Ground-based telescopes can only see the part of the sky above them at any given time. Space telescopes can be pointed in any direction.

Exam Tip: The three key advantages of space telescopes are: (1) no atmospheric absorption, (2) no atmospheric distortion, and (3) can observe 24/7 without weather interference. For full marks, explain WHY each advantage matters — e.g., "No atmospheric absorption means wavelengths like UV, X-ray and gamma rays can be detected, which are absorbed by the atmosphere and cannot reach ground-based telescopes."

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Disadvantages of Space Telescopes

Disadvantage	Explanation
Very expensive	Costs billions of pounds/dollars to build, launch and operate
Difficult to repair	If something goes wrong, a space mission is needed to fix it (e.g., the Hubble Space Telescope repair missions by Space Shuttle)
Limited lifespan	Components degrade in the harsh space environment (radiation, micrometeorites)
Size limitations	Must fit inside a rocket — ground-based telescopes can be much larger

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Absorption Spectra — What Are Stars Made Of?

One of the most powerful techniques in astronomy is spectroscopy — analysing the spectrum of light from a star to determine its chemical composition.

How It Works

1. A star emits light across a continuous spectrum of wavelengths (a rainbow of colours from red to violet).
2. As this light passes through the star's cooler outer atmosphere, certain wavelengths are absorbed by atoms of specific elements.
3. Each element absorbs light at characteristic wavelengths — unique to that element (like a fingerprint).
4. The result is an absorption spectrum: a continuous spectrum with dark lines (gaps) at specific wavelengths.

Reading an Absorption Spectrum

Feature	Description
Continuous spectrum	The rainbow background — all wavelengths present
Dark lines	Specific wavelengths absorbed by elements in the star's atmosphere
Pattern of lines	Unique to each element — acts as a chemical fingerprint
Matching	Compare the dark lines with known laboratory spectra of elements to identify what the star contains

Example

If the absorption spectrum of a star shows dark lines at the wavelengths corresponding to hydrogen, helium, sodium and calcium, then we know the star's atmosphere contains these elements.

Exam Tip: A common exam question asks how astronomers know what elements a star contains. The answer is: by analysing the star's absorption spectrum — the dark lines correspond to specific elements, and by comparing with laboratory spectra, the elements can be identified.

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Measuring Distance — The Light-Year

What Is a Light-Year?

A light-year is the distance that light travels in one year in a vacuum.

Quantity	Value
Speed of light	3 × 10⁸ m/s (300,000 km/s)
1 light-year	Approximately 9.46 × 10¹² km (about 9.5 trillion km)

A light-year is a unit of distance, not time (despite the word "year" in its name).

Why Use Light-Years?

Distances in the universe are so enormous that using kilometres or miles would result in impractically large numbers:

Distance	In km	In Light-Years
Earth to the Sun	1.5 × 10⁸ km	~8.3 light-minutes
Sun to nearest star (Proxima Centauri)	~4 × 10¹³ km	4.24 light-years
Across the Milky Way	~9.5 × 10¹⁷ km	~100,000 light-years
To the Andromeda galaxy	~2.4 × 10¹⁹ km	~2.5 million light-years

Exam Tip: Remember that a light-year is a unit of DISTANCE, not time. If asked to define it, say: "A light-year is the distance light travels in one year, approximately 9.46 × 10¹² km."

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Measuring Distance — Parallax

What Is Parallax?

Parallax is a method used to measure the distance to nearby stars. It uses the apparent shift in position of a star when viewed from two different positions in Earth's orbit around the Sun.

How It Works

1. Observe a nearby star from Earth at one point in its orbit (e.g., January).
2. Wait six months and observe the same star from the opposite side of Earth's orbit (e.g., July).
3. The nearby star appears to have shifted position slightly against the background of very distant stars.
4. The angle of this apparent shift is called the parallax angle.
5. The smaller the parallax angle, the further away the star is.

Limitations of Parallax

• Parallax only works for relatively nearby stars — the parallax angle becomes too small to measure for very distant stars.
• For more distant objects, astronomers use other methods such as standard candles (e.g., Type Ia supernovae) and red-shift (Hubble's law).

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