Assume that you've taken a spectrum of a star, and you'd like to estimate the star's size. Can you do it? Yes, if you also have a parallax angle for the star. This post quickly ties together different concepts to explain how observations (spectrum and parallax) can be used to estimate the size of a … Continue reading Stellar Radii and Stellar Spectra

You can estimate a star's size if you know its temperature and luminosity. To do this, assume that the surface of the star is a giant blackbody, then apply the Stefan-Boltzmann Law. The Stefan-Boltzmann law is discussed in section 5.2 of OpenStax Astronomy. To summarize, it states that the luminosity (total energy output as light) … Continue reading Stellar Radii and the Stefan-Boltzmann Law

https://youtu.be/HEheh1BH34Q One of the fundamental properties of a star is its size. In the next few weeks, we'll learn that a star's size is a hint at the processes taking place inside of it. For example, toward the end of their lives, the nuclear processes inside a star start to change. The new nuclear processes … Continue reading Measuring a Star’s Size – The Challenge

The entry on apparent and absolute magnitude left two (related) questions unanswered. How do we find the distance to a star?What is a parsec? You might also be wondering if learning the luminosity of a star is worth doing logarithms. The answer to that is yes, because knowing the luminosity of many stars led us … Continue reading Parallax

Warning: Math Ahead. Don't Panic. Our goal is to find the luminosity of and distance to a star. We know we can't find both, or even either, of these properties by observing only the brightness of a star. We also know that the problem is that stars come in a wide range of luminosity and … Continue reading Apparent and Absolute Magnitude

Before proceeding, I need to introduce the magnitude system (which is also discussed in your textbook). This system was developed in Ancient Greece, before telescopes or cameras or CCDs; so it may seem a little outdated.  It's still in very common use among astronomers, however, probably because it is a very human system. Imagine doing … Continue reading A Brief Introduction to the Magnitude System

Rigil Kentaurus A is one of the brightest stars in the sky, along with Sirius, Arcturus, Vega, and Rigel.  Unlike these other bright stars, which are all more luminous than the Sun, Rigil Kentaurus A is surprisingly Sun-like. Why is this star so bright when seen from the Earth?  Because it's also one of the … Continue reading Bright Stars – Luminous or Nearby?