How far away is Betelgeuse, the famously doomed star?

How to measure the distance from Betelgeuse to Earth

Introduction: The ALMA telescope in Chile has captured an image of the red giant star Betelgeuse at submillimeter wavelengths. The image shows something we have almost never seen before: a jet of hot gas is ejected from the atmosphere of the red giant star to the left. Through the ALMA telescope, we see extremely unstable red giant stars.

How far away is Betelgeuse, the star doomed to explode?

Betelgeuse is a bright red star in the constellation of Orion. It is at the end of its stellar life and one day it will explode as a supernova. In late 2019 and early 2020, Betelgeuse suddenly and unexpectedly dimmed. Some people joked that it would explode soon! But it didn't, but what if it did? When Betelgeuse becomes a supernova, will it affect life on Earth? How far away is Betelgeuse from us?

The good news is that if Betelgeuse did explode, it would be close enough to give us a spectacular light show, but far enough away not to cause any harm to Earth. First, let's answer the distance question. Betelgeuse is about 724 light-years away. In fact, getting this answer is very difficult, even for a star as close as Betelgeuse.

New measurement technology

Only in the last 30 years have astronomers been able to obtain more precise measurements of the distances to Betelgeuse and other nearby stars. New technology explains this progress. The technology began in 1989, when the space agency (ESA) launched a space telescope called Hipparcos (named after the famous Greek astronomer Hipparchus). After several years of observations, the Hipparcos space telescope provided parallax and distance data for more than 100,000 relatively nearby stars.

These measurements became the basis for most of the estimated stellar distances you see today.

Hipparcos's original data gave a parallax of 7.63 milliarcseconds for Betelgeuse; this is about one millionth the width of the full Moon. Calculations based on the parallax yielded a distance of about 430 light years.

The position of a nearby star relative to a distant background star changes slightly when viewed from 2 locations, and for Earth observations 6 months apart, the distance between the two locations is the diameter of the Earth's orbit. α is the parallax angle.

Measurement error

Because Betelgeuse is a variable star, which means its brightness fluctuates over time, we are very happy to see Betelgeuse dimming because it is the largest drop in brightness ever observed, but it also makes it more difficult to measure Betelgeuse's distance.

Subsequent studies have found some errors in Hipparcos' method of testing variable star data, which led to a smaller test distance for Hipparcos. After correcting these errors, astronomers gave a parallax of 5.07 milliarcseconds, which changed the estimated distance of Betelgeuse from the initial measurement of 430 light-years to about 643 light-years (with an error of ±46 light-years).

In 2017, astronomers published new calculations that further confirmed Betelgeuse's parallax to 4.51 milliarcseconds. This new analysis of the Hipparcos data also included observations from several ground-based radio telescopes. This puts Betelgeuse's test distance at about 724 light-years, or more precisely, taking into account some uncertainties in the data, between 613 and 881 light-years.

Why Gaia can't measure the distance to Betelgeuse

As you may know, the goal of the space agency's Gaia astrometry mission is to map the Milky Way in three dimensions. With the release of Gaia's Wave 3 data in June 2022, the ESA says there are now an estimated 2 billion stars in the Milky Way.

However, Betelgeuse is not among these stars, and astronomers cannot use Gaia to find a more precise distance for Betelgeuse, primarily because Betelgeuse is too bright for the spacecraft's sensors.

Map of Orion the Hunter, showing the location of Betelgeuse

More about parallax

Have you ever looked at a nearby object from two different locations and noticed how its position relative to a distant landmark changes? This is called the parallax effect. Astronomers measure the position of a distant background star in the sky from Earth six months apart, with the nearby star as the center point. During that time, Earth has moved to the other side of its orbit. Therefore, the two locations are about 186 million miles (300 million kilometers) apart, or the diameter of Earth's orbit. Using the difference in the observed star's position relative to the background stars at the two locations, astronomers are able to derive the parallax angle and calculate its distance from Earth.

The concept of parallax was known to ancient Greek astronomers, but they lacked the technology to make very fine angular measurements in the sky. As a result, all attempts to measure stellar parallax failed until German astronomer Friedrich Bessel succeeded in 1838. He used a telescope, and although his two observing positions were on opposite sides of Earth's orbit, he had difficulty discerning the tiny angular displacement, but it was enough for him to determine the distance of the star 61 Cygni to be 11 light years.

Limitations of using parallax

From Bessel's time until the launch of Hipparcos in 1989, astronomers compiled only a few thousand parallaxes. The process is difficult because of a multitude of factors. The tiny angles involved, instrumental imperfections, and the cloudiness of Earth's own atmosphere all hamper the measurements. The atmosphere distorts observations from Earth, even from very clear and dark places like deserts and mountaintops.

Hipparcos began taking observations from space in 1989, breaking through the limitations of the Earth's atmosphere to obtain data on the positions of stars with unprecedented accuracy at the time. Astronomers are continuing to refine these measurements through innovations in instrumentation and data analysis methods, using a combination of ground-based and space-based observatories.

Conclusion: Measuring the distance to Betelgeuse is particularly difficult because it is a variable star. Complex calculations based on data from the Hipparcos Space Telescope and ground-based radio telescopes indicate that it is about 724 light-years away.

Want to keep track of how bright or dim Betelgeuse is on any given day? Follow the Betelgeuse Status account on Mastodon.

BY:Editors of EarthSkyand

FY: People

If there is any infringement of related content, please contact the author to delete it after the work is published.

Please obtain authorization for reprinting, and pay attention to maintaining integrity and indicating the source