# Hottest Star in the Universe

What is the hottest star in the universe? With a surface temperature thirty-five times hotter than our sun, WR 102 tips the scales at 50,000 ℉ and is the hottest star in the universe.

Understanding how and why WR 102 is the hottest star in the universe requires that we delve into the evolution and composition of a star.

## Lifecycle of a Star

A star has a lifecycle during which it will form, grow, shrink and finally expire. Different stars follow different growth paths depending on their formation mass. Understanding how stars are classified aids in our specific knowledge of WR 102.

## Hertzsprung-Russell Diagram

Danish astronomer Ejnar Hertzspring began working on star classification charts in the early 1900s. Hertzspring plotted the color of a star against its brightness. Henry Russel, a US astronomer, also worked independently to develop the same chart type.

Today we know the charts as the Hertzsprung-Russell diagram. The chart plots the data from many stars into an easy-to-understand diagram.

The diagram above helps to rapidly understand a star’s individual properties when its luminosity is plotted against its surface temperature.

• Luminosity

### It is calculated from the stars’ brightness and the stars’ distance from earth.

• The luminosity value allows for awareness and mathematical correction that two stars may be the same size and age but have different brightness levels due to differing distances from earth.
• The luminosity scale (left vertical axis) runs from 10-5 to 106. Objects with lower luminosity will be lower in value (10-5), and objects with higher luminosity will be higher in value (106.)
• Brightness

A Charge Coupled Device (CCD) is attached to a telescope, and the intensity of the light emitted from the star is measured.

• Example: The measured intensity from a lighthouse lamp has a very high brightness value when you’re fifty yards away in a boat. The same lighthouse, with the same light, has different intensity values when measured from five miles away. The size of the lamp hasn’t changed, but the distance from the lamp decreases the lamp’s measured brightness.
• Kelvin

A unit of measurement used in space science and astronomy.

• On earth, we’re evaluating “moderate” temperatures that we might expect to encounter. Our units of temperature measurements are tied to those expectations. (Extreme temperatures of -130℉/-54℃ in Antarctica and 134℉/57℃ in Death Valley.)
• In space, the temperatures are much more extreme, thousands of degrees in variation.
• Kelvin measurements begin at absolute zero. There are no negative values in a Kelvin measurement. All values are 0 and greater.
• Kelvin=0
• Celsius= −273.15℃ (zero at Kelvin)
• Fahrenheit −459.67℉ (zero at Kelvin)
• The use of Kelvin measurements standardizes the measurement unit.

The Kelvin axis is plotted from 30,000 to 3,000 on this chart. (Large to small.) The far left value of 30,000 equates to the highest temperature (blue). The coldest value (red) corresponds to the lowest temperature (3,000).

Hint: Think about a gas furnace or a gas oven in the kitchen. We want the flame to be blue! Blue is hotter than red!

• What do the different quadrants mean?

Each star’s temperature and luminosity value are entered into a scatterplot. Each section of the scatterplot (the Hertzsprung-Russell diagram) can be analyzed.

• Lower Left-Lowest luminosity and the highest temperature
• Upper Left-Highest luminosity and the highest temperature
• Lower Right-Lowest luminosity and the lowest temperature
• Upper Right-Highest luminosity and the lowest temperature

## Types of Stars

The geographic position of a star in the Hertzsprung-Russell diagram indicates its position in its lifecycle. The mass of a star when it’s “born” will impact which lifecycle a star passes through.

• White Dwarfs-
• The end of the “evolution line” for low and intermediate-mass stars.
•  White-colored with a mass similar to our sun and roughly earth’s size.
• A star burns off all of its hydrogens during the “main sequence” of their lives.
• After cooling, the star expands into a Giant or Super Giant
• All of the Giant’s mass is blown off.
• A small compressed core, named “White Dwarf,” is left behind.
• Sequence-
• Ninety percent of the stars in the universe are sequence stars.
• Starting life as a scorching hot and bright star in the upper left quadrant of the Hertzsprung-Russell diagram
• Progresses through the center of the graph (where you’ll find our sun) and ends in the lower right corner of the diagram as a cooler and dimmer star.
• The bigger the mass, the shorter the lifespan. Stars with higher mass consume their fuel faster. Smaller mass stars (red dwarfs) “live” longer.
• Super Giants-
• Gigantic stars that may be up to three hundred times the size of the sun
• High luminosity with values approximately 1,000,000 times greater than the sun.
• Short life span; burning bright and fast, the star may pass through its entire lifecycle in a few million years. Our sun will last as a main sequence star for approximately twenty billion years.
• Giants
• Slightly smaller radius than Super Giants
• Lower luminosity than Super Giants
• Lower temperature than Super Giants

## Hottest Star in the Universe: Wolf-Rayet

In 1867 Charles Wolf and George Ranet observed an unusual pattern in the light wavelength from a star. A distinctive pattern in the wavelength led to the discovery that the star was emitting helium instead of hydrogen.

WR stars make up just a tiny percentage of all the stars in the universe. There are less than 500 known WR stars in the Milky Way and fewer than 300 in our surrounding galaxies.

In  Wolf-Rayet stars, the outer helium elements are blown off the star and leave the carbon, oxygen, and nitrogen in their atmospheres. There are elements that are speeding away from the star on Stellar winds traveling at approximately 4.5 million mph (7.3 million km/h)

The remaining elements burn at approximately 200,000 Kelvin. Wolf-Rayet stars are the hottest stars in the universe. WR 102 has a surface temperature about thirty-five times greater than the sun. Wolf-Rayet stars are in their final act before turning into a SuperNova.

## Final Act

WR 102 is in the final stages of its life cycle as it (quite literally) tears itself apart. Scientists are fascinated as they watch the hottest star in the galaxy complete its transition. Astronomers believe the metamorphosis will end within the next 1,500 years.