Tools Used in Astronomy

Astronomy Notes

1. Electromagnetic Spectrum

Definition

The electromagnetic spectrum is the complete range of all electromagnetic radiation in the universe.

Types include:

• Gamma rays

• X-rays

• Ultraviolet (UV)

• Visible light

• Infrared (IR)

• Microwaves

• Radio waves

Key Properties of Light

Frequency

• Number of wave cycles passing a point per second

• Measured in Hertz (Hz)

Wavelength

• Distance between two consecutive wave peaks

• Measured in meters

Relationship Between Frequency, Wavelength, and Energy

• Higher frequency → shorter wavelength → higher energy

• Lower frequency → longer wavelength → lower energy

Visible Light

• Only part of the EM spectrum detectable by human eyes

• Different wavelengths correspond to different colors

Star Colors

• Red stars = cooler

• Blue stars = hotter

Spectrum Regions and Uses

Atmosphere and Observation

Earth’s atmosphere blocks much EM radiation:

• X-rays

• Gamma rays

• Most UV

• Parts of infrared

Therefore:

• Space telescopes are needed for many wavelengths.

2. Telescopes

Definition

A telescope collects, focuses, and magnifies electromagnetic radiation.

Main Types of Optical Telescopes

Refracting Telescope

Uses lenses.

Structure

• Objective lens gathers light

• Light converges at focal point

• Eyepiece magnifies image

Advantages

• Sharp stable images

• Sealed tube prevents air disturbance

• Minimal maintenance

Disadvantages

• Chromatic aberration

• Expensive high-quality lenses required

Chromatic Aberration

Different wavelengths bend differently through lenses, causing colored edges around objects.

Reflecting Telescope

Uses mirrors.

Invented by Isaac Newton.

Structure

• Concave primary mirror gathers light

• Secondary mirror redirects light to eyepiece

Advantages

• No chromatic aberration

• Compact design

• Cheaper than refractors

Disadvantages

• Requires cleaning

• Exposed tube affected by environment

3. Other Telescope Types

Radio Telescopes

• Detect radio waves

• Large dish structures

• Used for faint long-wavelength signals

Infrared Telescopes

• Usually placed on mountains

• Avoid water vapor interference

Ultraviolet, X-ray, Gamma-Ray Telescopes

• Usually space-based

• Atmosphere absorbs these wavelengths

4. Inverse Square Law

Definition

Light intensity decreases with distance.

Formula

I∝1d2I \propto \frac{1}{d^2}I∝d21​

Where:

• III = intensity

• ddd = distance

Example

• Double the distance → intensity becomes 1/4

• Triple the distance → intensity becomes 1/9

5. Telescope Powers

Light-Gathering Power

Ability to collect light.

Important Idea

Larger objective diameter = more collected light.

Objective Area Formula

A=π4D2A=\frac{\pi}{4}D^2A=4π​D2

Where:

• AAA = area

• DDD = diameter

Resolving Power

Ability to distinguish fine details or separate close objects.

Formula

θR∼λD\theta_R \sim \frac{\lambda}{D}θR​∼Dλ​

Where:

• θR\theta_RθR​ = minimum resolvable angle

• λ\lambdaλ = wavelength

• DDD = objective diameter

Important Ideas

• Larger diameter → better resolution

• Longer wavelength → worse resolution

Magnifying Power

Ratio between focal lengths.

Formula

M=fofeM=\frac{f_o}{f_e}M=fe​fo​​

Where:

• MMM = magnification

• fof_ofo​ = focal length of objective

• fef_efe​ = focal length of eyepiece

Notes

• High magnification also magnifies distortions

• Image becomes dimmer at extreme magnification

6. Observatories

Definition

Facilities containing telescopes and astronomical instruments.

Ground-Based Observatories

Located:

• Far from cities

• High altitudes

• Dry climates

Examples

• Mauna Kea Observatory

• Kitt Peak National Observatory

• Paranal Observatory

Radio Observatories

Usually placed in valleys to reduce interference.

Space-Based Observatories

Advantages:

• No atmospheric distortion

• Observe all wavelengths

Disadvantages:

• Extremely expensive

Example

Chandra X-ray Observatory

Airborne Observatories

Mounted on aircraft or balloons.

Purpose:

• Reduce interference from atmosphere and water vapor

7. Spectroscopy

Definition

Study of spectra to determine:

• Composition

• Temperature

• Motion

• Physical properties

8. Types of Spectra

Continuous Spectrum

Produced by:

• Dense hot objects

• Stars

• Black bodies

Contains all wavelengths continuously.

Emission Spectrum

Bright lines on dark background.

Produced by:

• Hot low-density gases

Electrons release photons when dropping energy levels.

Absorption Spectrum

Dark lines on bright background.

Produced when:

• Continuous light passes through cooler gas

Specific wavelengths are absorbed.

9. Kirchhoff’s Laws

Developed by Gustav Kirchhoff.

1. Black body emits continuous spectrum

2. Hot low-density gas emits bright-line spectrum

3. Cool gas in front of continuous source creates absorption lines

10. Blackbody Radiation

Black Body

Ideal object absorbing and re-emitting all radiation.

Blackbody Curve

Graph of:

• Wavelength vs intensity

Temperature Effects

• Higher temperature → shorter peak wavelength

• Lower temperature → longer peak wavelength

11. Wien’s Law

Relates temperature and peak wavelength.

λmaxT=2.898×10−3 m\cdotpK\lambda_{max}T = 2.898 \times 10^{-3}\ \text{m·K}λmax​T=2.898×10−3 m\cdotpK

Where:

• λmax\lambda_{max}λmax​ = wavelength of maximum emission

• TTT = temperature in Kelvin

Applications

Used to determine star temperatures.

12. Doppler Effect

Definition

Change in observed wavelength due to motion.

Redshift

Object moving away:

• Wavelength increases

• Spectrum shifts toward red

Blueshift

Object moving toward observer:

• Wavelength decreases

• Spectrum shifts toward blue

Uses

Determines:

• Direction of motion

• Relative velocity

• Stellar rotation

13. Spectroscopy and Composition

Every element has unique spectral lines.

If spectral lines match an element:

• That element exists in the star.

Used to determine:

• Chemical composition

• Element abundance

• Stellar atmospheres