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Applications and Skills:
Sketching and interpreting HR diagrams
Identifying the main regions of the HR diagram and describing the main properties of stars in these regions
Applying the mass–luminosity relation
Sketching and interpreting evolutionary paths of stars on an HR diagram
Describing the evolution of stars off the main sequence
Describing the role of mass in stellar evolution
Building an HR Diagram
Building an HR Diagram
Cut out all the stars.
Look for patterns within the stars, how can you cluster them into groups?
Best by temperature?
Best by color?
Luminosity?
How to Space your Stars
HR Diagrams and Logarithmic Scales
HR Diagrams and Logarithmic Scales
Hertzsprung-Russell Diagrams
Hertzsprung-Russell Diagrams
This graph shows the absolute luminosity of almost one million stars observed by Gaia as a function of their colour. Astronomers call it a ‘Hertzsprung-Russell diagram’, named for the two early 20th century scientists who recognised that such a diagram could be used as a tool to understand stellar evolution.
The diagram is based on a combination of data from Gaia's first year of observations and earlier data from ground-and space-based telescopes.
To estimate a star's true brightness or ‘absolute luminosity’, astronomers need to know its distance. Distances are estimated from parallax, the apparent motion of a star against a distant background observed over the period of a year and resulting from the Earth's real motion around the Sun; this is also observed by Gaia as it orbits the Sun alongside Earth.
But parallax is not the only movement seen by Gaia: the stars are also really moving through space, which is called proper motion.
Gaia has made an average of roughly 14 measurements of each star on the sky thus far, but this is generally not enough to disentangle the parallax and proper motions. To overcome this, the scientists have combined Gaia data with positions extracted from the Tycho-2 catalogue, based on data taken between 1989 and 1993 by Gaia's predecessor, the Hipparcos satellite.
The luminosity measurements are based on data from Hipparcos and ground-based telescopes, and the colour information comes also from ground-based observations.
This preliminary diagram provides a taste of what the mission will deliver in the coming years. Later it will be possible to compile a ‘Hertzsprung-Russell diagram’ based on the Gaia data alone.
The data points appear to populate some characteristic regions of the diagram, with most of them distributed along the diagonal running from the top left corner to the bottom right: this is called the main sequence of stars, identifying all stars that are burning hydrogen in their cores – a phase that takes up the majority of a star's lifetime. Along the main sequence, brighter and more massive stars are located towards the top left of the diagram, whereas stars with lower masses and brightnesses are found towards the lower right.
The large clump of data points in the right half of the graph identifies red giant stars: these are evolved stars that have exhausted hydrogen in their cores. As their cores collapse under their own weight, the outer layers of these stars inflate, creating large and cool – thus red – envelopes.
Reading H-R Diagrams
Reading H-R Diagrams
A useful tool developed by Ejnar Hertzsprung and Henry Russell is the HertzsprungRussell diagram where stars in our galaxy are plotted on a luminosity-temperature axis. The temperature scale goes from coolest on the right to hottest on the left while the luminosity scale is dimmest at the bottom and brighter at the top. This paces cooler, dimmer stars towards the lower right and the hotter, brighter stars towards the upper left. The diagonal line between these two extremes is called the main sequence. Our Sun is nearly in the middle of both the temperature and luminosity scales and is therefore around the middle of the main sequence. Stars above the main sequence with the same temperature as cooler main sequence stars have greater surface areas. These enormous stars are called Red Giants. An example of a Red Giant is Antares. Its surface temperature is cooler than the Sun but its luminosity is about 50,000 times brighter, which means that it has a very large radius. Stars that have the same luminosity as dimmer main sequence stars but are to the left of them (hotter) have smaller surface areas. These small, hot stars are called White Dwarfs. They can have a radius as similar to the Earth but have temperatures of around 10,000 K.
luminosity (noun)
the total amount of energy emitted by a star or other celestial object per unit of time.
Example Sentence: The luminosity of the star indicates its brightness.
scatter plot (noun)
a graph in which the values of two variables are plotted along two axes, with each data point represented as a dot.
Example Sentence: The scatter plot showed the relationship between temperature and brightness of the stars.
evolution (noun)
the gradual development and change of something over time.
Example Sentence: The H-R diagram helps us understand the evolution of stars.
main sequence (noun)
a stage in the life cycle of a star during which it burns hydrogen into helium in its core.
Example Sentence: Most stars spend the majority of their lives in the main sequence.
white dwarf (noun)
a small, dense star that is the remnant of a low-mass star after it has exhausted its nuclear fuel.
Example Sentence: The white dwarf is the final stage of evolution for low-mass stars.
In the Stefan-Boltzmann law L=σAT4if the radius R and therefore the surface area A is kept constant then the luminosity can be plotted against temperature to produce a set of diagonal lines called the lines of constant radius. In the HR diagram below the Sun is marked with a red cross.
1. Who were the astronomers who created the Hertzsprung-Russell diagram?
Answer:
The astronomers who created the Hertzsprung-Russell diagram were Ejnar Hertzsprung and Henry Norris Russell.
2. What is the main purpose of the Hertzsprung-Russell diagram?
Answer:
The main purpose of the Hertzsprung-Russell diagram is to help astronomers classify stars based on their characteristics.
3. What can astronomers determine by studying a star's position on the Hertzsprung-Russell diagram?
Answer:
By studying a star's position on the Hertzsprung-Russell diagram, astronomers can determine its internal structure, evolutionary stage, and other characteristics.
4. Based on the information in the text, what can be inferred about the relationship between a star's temperature and its luminosity?
Answer:
Based on the information in the text, it can be inferred that there is a relationship between a star's temperature and its luminosity. The Hertzsprung-Russell diagram shows that hotter stars tend to be more luminous, while cooler stars are less luminous.
5. Evaluate the significance of the Hertzsprung-Russell diagram in understanding the life cycle of stars.
Answer:
The Hertzsprung-Russell diagram is significant in understanding the life cycle of stars. By studying a star's position on the diagram, astronomers can determine its evolutionary stage and other characteristics. This helps us understand how stars change over time and provides insights into their internal structure and properties.
6. Analyze the importance of the Hertzsprung-Russell diagram in classifying stars compared to other methods used by astronomers.
Answer:
The Hertzsprung-Russell diagram is highly important in classifying stars compared to other methods used by astronomers. It provides a visual representation of the relationship between a star's temperature and luminosity, which allows for easy classification based on their characteristics. Other methods may require more complex calculations or observations, making the Hertzsprung-Russell diagram a valuable tool in the field of astronomy.
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