Hertzsprung Gap
The Hertzsprung Gap is a notable feature within the Hertzsprung-Russell diagram (H-R diagram), which is a fundamental tool in astronomy for plotting stars based on their brightness and temperature. This gap is observed between the main sequence of stars and the regions occupied by red giants and supergiants, where there are significantly fewer stars present. The H-R diagram classifies stars in various stages of their life cycles, with most stars, including our Sun, found on the main sequence. In contrast, the Hertzsprung Gap represents a phase where stars transition from burning hydrogen in their cores to burning hydrogen in a shell surrounding the core. During this transition, these stars become cooler and dimmer, making them harder to detect, which contributes to the apparent scarcity of stars in this region. The gap was named after Ejnar Hertzsprung, who first noted this lack of stars in that area. Understanding the Hertzsprung Gap provides insights into stellar evolution and the lifecycle of stars, enhancing our knowledge of the universe.
Hertzsprung Gap
FIELDS OF STUDY: Astronomy; Astrophysics; Stellar Astronomy
ABSTRACT: Astronomers use the Hertzsprung-Russell diagram to chart the evolution of stars. The Hertzsprung gap is a section of this diagram where few stars are found. The Hertzsprung gap is important because it defines the stage in a star’s evolution in which it transitions from burning fuel in its core to burning fuel in the shell around its core.
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram (H-R diagram), devised in the early twentieth century by Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957), is a chart that plots the brightness of stars against their temperature. It provides a way of categorizing stars based on where they are in their life span. Scientists can use the H-R diagram to study the age and evolutionary stage of different stars.
Stars plotted in the H-R diagram fall into several main areas. One such area is the main sequence, which follows a gentle diagonal curve from left to right. It begins with the hot, bright stars in the upper-left corner and runs down to the lower-right corner of the square, where the cooler, dimmer stars are found. Most stars, including the sun, are found in the main sequence. The red giants and supergiants are found in the upper-right corner of the H-R diagram. These stars are brighter but have lower surface temperatures than other stars. White dwarves are found in the lower-left corner. These stars are hot but small and not as bright as other stars.
There is a "gap" in the H-R diagram, between the main sequence and the red giants and supergiants, where very few of the observed stars in the universe can be found. However, this gap is not truly empty. Stars in the main sequence burn hydrogen in their core, resulting in their hot, bright status. Red giants and supergiants have exhausted the hydrogen in their core and have begun to burn the shell surrounding the core, causing them to be brighter but lower in temperature. When main-sequence stars are making the transition from burning the core to burning the shell, however, they are both dimmer and cooler, making them difficult to see. These are the stars that fall into the Hertzsprung gap. Because Hertzsprung was the first scientist to notice the lack of apparent stars in this area of the diagram, the gap was named for him.
Diagramming the Stars
Scientists compare the characteristics of similar objects to either determine if the objects are related or identify tendencies in their actions and reactions. In the early 1910s, Hertzsprung and Russell each independently devised a method for charting the characteristics of stars to make such comparisons easier. Hertzsprung created a chart comparing the luminosity (total energy output of an astronomical object) of stars to their temperature (as represented by color) to see how the two characteristics were related, while Russell plotted the stars’ absolute magnitude (intrinsic brightness) against their spectral type. The spectral type of a star is determined by spectral classification,which categorizes stars based on the spectrum of electromagnetic radiation they emit.
The two methods produced similar charts. This is because magnitude is a function of luminosity, and a star’s emission spectrum is determined mainly by its temperature. When scientists plot stars and star clusters in the H-R diagram, they use relative temperature, brightness, and color to determine both the age of the star or cluster and where it is in its evolution.
Stars may appear in one of two types of star clusters: open clusters or globular clusters. Open clusters are groups of stars that are separate enough for an observer with a telescope to see them as individuals. A few open clusters, including the Pleiades and the Hyades, are close enough to Earth to be seen without a telescope. These open clusters are also called galactic clusters because they are found on the arms of spiral galaxies. While gravity holds the cluster together, it is weak. Members of the cluster can be ejected from the cluster by the gravitational force of other space objects.
Globular clusters are groups of stars found mainly in the halo surrounding the galaxy. They are much older than other stars. The gravitational forces that hold globular clusters together are strong, so these clusters do not usually lose their stars to other forces.
What Scientists Learn from Plotting Stars
Scientists know that all stars follow a similar pattern throughout their lifetime. The stages of this evolution are determined by the star’s composition and how it burns matter to create light and energy. If they know the temperature and luminosity of a star, scientists can determine both its age and its stage in life.
The H-R diagram helps with this determination. As stars age, they move across the diagram over thousands or tens of thousands of years. Knowing if a star is in the main sequence, among the red giants or the white dwarfs, or in the Hertzsprung gap tells scientists more about a star than they could learn from simple observation.
PRINCIPAL TERMS
- Hertzsprung-Russell diagram: a tool used by scientists to classify and chart stars according to their temperature and luminosity.
- spectral classification: the categorization of stars according to the electromagnetic frequencies they emit.
- star cluster: a group of stars that share a common origin and are held together by gravity for a period of time.
Bibliography
Tsarevich, Omar, Azalea Bear, and Noam Poker. "Faint Intermediate Luminosity Optical Transients (I Lots) from Engulfing Exoplanets on the Hertzsprung Gap." Monthly Notices of the Royal Astronomical Society, vol. 511, no. 1, Mar. 2022, pp. 1330-1335, DOI: 10.1093/Asmara/stac081. Accessed 14 June 2022.
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