Main Sequence and the Hertzsprung-Russell Diagram
Most stars spend the bulk of their lives fusing hydrogen in their cores, which places them along a single diagonal band, the main sequence, on the diagram of luminosity against temperature that organizes all of stellar astronomy.
Definition
The main sequence is the band on the Hertzsprung-Russell diagram occupied by stars that generate their energy by fusing hydrogen into helium in their cores, and the diagram itself is a plot of stellar luminosity against surface temperature or color.
Scope
The topic covers the Hertzsprung-Russell diagram and its observational counterpart the color-magnitude diagram, the main sequence as the locus of core hydrogen burning, the mass-luminosity relation that orders stars along it, the zero-age main sequence and the main-sequence lifetime, and the use of the cluster main-sequence turnoff to determine ages.
Core questions
- Why do most stars lie along the main sequence?
- What property of a star fixes its location on the main sequence?
- How does the main-sequence lifetime depend on stellar mass?
- How is the age of a star cluster read from its main-sequence turnoff?
Key concepts
- main sequence
- Hertzsprung-Russell diagram
- color-magnitude diagram
- mass-luminosity relation
- zero-age main sequence
- main-sequence turnoff
- spectral type
Key theories
- The main sequence as core hydrogen burning
- Stars settle onto the main sequence when steady hydrogen fusion in the core supplies their luminosity in hydrostatic and thermal balance; because this is the longest-lived stable phase, the main sequence is where most stars are found at any time.
- Mass-luminosity relation and turnoff ages
- Along the main sequence luminosity rises steeply with mass, so massive stars are brighter but shorter-lived; in a coeval cluster the most massive surviving stars define a turnoff point whose location gives the cluster's age.
Mechanisms
A newly formed star contracts until its core is hot enough for hydrogen fusion, at which point it settles onto the zero-age main sequence with a luminosity set mainly by its mass. It remains there, slowly brightening, until core hydrogen is exhausted, after which it leaves the main sequence; in a cluster this exhaustion proceeds from the most massive stars downward, sweeping the turnoff to lower masses over time.
Clinical relevance
The main sequence is the backbone of stellar classification and a primary tool for measuring ages and distances: main-sequence fitting calibrates cluster distances, turnoff ages date stellar populations and the Galaxy, and the diagram organizes the comparison of theory with observed stellar populations.
History
Hertzsprung and Russell independently discovered the luminosity-temperature relation in the early 1910s, building on the spectral classification of Cannon; Payne-Gaposchkin later showed that stars are made chiefly of hydrogen, explaining why hydrogen burning defines the main sequence.
Key figures
- Ejnar Hertzsprung
- Henry Norris Russell
- Annie Jump Cannon
- Cecilia Payne-Gaposchkin
Related topics
Seminal works
- russell1914
- hertzsprung1911
Frequently asked questions
- Is the main sequence an evolutionary track?
- No, it is not a path that a single star moves along; it is the set of positions occupied by many different stars while they burn hydrogen in their cores, with each star's place fixed mainly by its mass and changing only slowly during this phase.
- How can the HR diagram of a cluster give its age?
- All stars in a cluster formed together, so the most massive ones exhaust their core hydrogen first and peel off the main sequence; the mass at which stars are just leaving, the turnoff, corresponds to a known lifetime that equals the cluster's age.