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
Astronomy 112
Stellar Radii and the Stefan-Boltzmann Law
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
Measuring a Star’s Size – The Challenge
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
Parallax
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
Apparent and Absolute Magnitude
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
A Brief Introduction to the Magnitude System
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 This system was developed in Ancient Greece, before telescopes or cameras or CCDs; so it may seem a little outdated.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
Bright Stars – Luminous or Nearby?
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?
Celestial Coordinates
The celestial sphere is a tool that astronomers use to map the sky. To navigate the map, we need coordinates. Maps of the Earth have longitude and latitude; the celestial sphere has right ascension (RA) and declination (Dec). To make sense of longitude and latitude, you first need to locate the North and South Poles. … Continue reading Celestial Coordinates
Celestial Sphere
Astronomers map the sky. Maps of the sky have aided navigators for thousands of years. They also let astronomers locate and identify astronomical objects such as stars, nebulae, and galaxies and plan astronomical observations. Backyard astronomers use sky atlases (likely digital) to find interesting objects in the sky. Professional astronomers map the sky so that … Continue reading Celestial Sphere
Confidence
Confidence is a tricky concept, but it's also a pragmatic one. We can never be 100% sure of anything, but we still need to get on with the business of living our lives. There are bridges and airplanes and smartphones to build, and they all rely our current scientific theories. We don't have certainty that … Continue reading Confidence
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