Module 9 / Lecture 1 : Apparent Brightness, Luminosity, and Distance

Summary

This lecture covers the concepts of apparent brightness, luminosity, and distance of stars. It explains that a star's apparent brightness is how bright it appears to us, while luminosity is the total power it emits. The relationship between distance and brightness is described using the inverse square law. The magnitude system is introduced, including both apparent and absolute magnitudes, to compare stars' brightness. Stellar parallax is discussed as a method to measure star distances, which in turn helps determine their luminosity. Further topics like stellar temperature and mass will be covered in future lectures.

Takeaways

• 🌟 Apparent brightness and luminosity are two different things; brightness is how we perceive a star, while luminosity is the total light it emits.
• 🌍 The apparent brightness of a star depends on its distance and the amount of light it emits.
• 🔭 Example: Betelgeuse appears as bright as Procyon, though Betelgeuse emits much more light because Procyon is much closer.
• 💡 Apparent brightness is measured as the amount of power reaching us per square meter.
• 🔆 Luminosity refers to the total energy a star radiates into space.
• 📏 The inverse square law governs how apparent brightness diminishes with increasing distance.
• 🔢 The magnitude system classifies stars based on their apparent brightness; higher magnitudes indicate dimmer stars.
• 🌙 Objects brighter than magnitude zero have negative values; for instance, the full Moon has an apparent magnitude of -13.
• 🧑‍🚀 Parallax is a method used to determine the distance of nearby stars by measuring their apparent shift in position over six months.
• 📐 The distance to a star in parsecs can be calculated using its parallax angle in arcseconds.

Q & A

What is the difference between apparent brightness and luminosity?

Apparent brightness is how bright a star appears from Earth, while luminosity is the total amount of power a star emits into space.

Why do some stars appear brighter than others in the night sky?

The brightness of a star in the night sky depends on both its luminosity and its distance from Earth. A star may appear bright because it is very luminous or because it is close to Earth.

What example does the lecture give to explain the difference in brightness and luminosity?

The lecture compares Procyon and Betelgeuse, which appear equally bright in the sky, but Betelgeuse emits 15,000 times more light than Procyon because it is much farther away.

How is apparent brightness defined?

Apparent brightness is defined as the amount of power reaching us per unit area, or per square meter.

What is the inverse square law of light?

The inverse square law of light states that the apparent brightness of a light source is inversely proportional to the square of its distance from the observer.

How does the lecture illustrate the inverse square law of light?

The lecture uses an example of counting photons on imaginary spheres around a star. Moving to a sphere twice as far away means each square meter receives only a quarter of the light.

What is the magnitude system and who developed it?

The magnitude system classifies stars based on their apparent brightness and was developed by the Greek astronomer Hipparchus over 2000 years ago.

How do astronomers denote apparent and absolute magnitudes?

Apparent magnitudes are denoted with a lower-case 'm' and absolute magnitudes with an upper-case 'M'.

What is the relationship between apparent magnitude numbers and brightness?

A larger number for apparent magnitude means a dimmer apparent brightness. For example, a star of apparent magnitude 4 appears dimmer than a star of magnitude 1.

What is stellar parallax and how is it used to measure distances?

Stellar parallax is the apparent shift in position of a nearby star against the background of distant stars, observed from two points in Earth's orbit. The parallax angle is used to calculate the star's distance.

How is the distance to a star related to its parallax angle?

The distance to a star in parsecs is equal to 1 divided by the parallax angle in arcseconds. For example, a star with a parallax angle of 0.5 arcseconds is 2 parsecs away.

Why couldn't ancient Greeks measure stellar parallax?

Ancient Greeks couldn't measure stellar parallax because even the nearest stars have parallax angles smaller than 1 arcsecond, which is below the angular resolution of the human eye.