Early Star: Young Big Brother? Or the Other Half of the “Magic Star”?

Produced by: Science Popularization China

Author: Guo Yanjun (Yunnan Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

The ancients once called a star that flickered constantly and glowed brightly and dimly as a "devil star". It was not until a deaf-mute boy from the UK, Goodricke, discovered its secret through long-term observation:

It turns out that this is a pair of "binary stars" leaning against each other, one bright and one dim. When the darker star blocks the "devil star", part of the light of the "devil star" will be blocked at the same time, so humans on Earth observe that the "devil star" has become darker. Conversely, when the dark star is blocked by the "devil star", we will see a brighter "devil star".

In fact, "devil stars" are not isolated cases in the universe, but a common phenomenon, especially for early-type stars. Most early-type stars belong to binary star systems.

Figure 1 Schematic diagram of binary star rotation

(Image source: space)

What is an “early-type star”?

The starlight seen by the boy tells a distant story. The optical spectrum of a star contains rich physical information about distant celestial bodies, which can be said to be the "fingerprint" and "DNA" of celestial bodies. When chemical substances burn, different elements in the substance will show unique spectra and colors. Therefore, astronomers can get the elemental composition from the spectrum of celestial bodies, and through further analysis, they can even get the mass, magnetic field, rotation, surface gravity and movement of the star.

Stars have different colors because of their different effective temperatures. The higher the effective temperature of a star, the bluer the color, and vice versa. According to the composition of the spectrum, in descending order of effective temperature, astronomers divide the star spectrum into O, B, A, F, G, K, M, R, N and S types. Usually, O, B, A type stars with higher temperatures are called "early type stars", and K, M type stars with lower temperatures are called "late type stars". The rest are called "intermediate type stars".

Figure 2 Schematic diagram of an OB-type early-type star

(Image source: Wikipedia)

The reason for this name is that people initially thought that temperature indicated the order of star formation, but later it was discovered that early-type stars and late-type stars do not represent the early or late formation of stars. However, stars also have a process of birth, maturity, aging and death, but this process often takes millions or even more than 10 billion years.

The mass of a star is related to its physical properties and determines its lifespan and evolution process. A star with a large mass evolves faster than a star with a small mass. Therefore, for stars of the same age, an early-type star may have left the main sequence and entered its "twilight years", while a mid-late-type star may still be in the main sequence and be in its "prime".

Why are early-type stars so important?

Here I would like to add a little knowledge point: Compact objects are small but dense objects, including white dwarfs, neutron stars and black holes. When two compact objects in the universe revolve around each other, they will produce gravitational waves that are invisible to the naked eye.

(Photo source: Veer Gallery)

Binary star systems composed of early-type stars may eventually evolve into gravitational wave sources such as binary neutron stars, black hole-neutron stars, and binary black holes. Therefore, the statistical properties of massive binaries are crucial for tracking the formation of massive stars and constraining the properties of binary star populations.

Black Hole

(Photo source: Veer Gallery)

LAMOST: No problem, I'll help.

At present, most of the early-type star observation samples come from different observations, and lack consistent observation samples, which leads to large deviations in the research results of the statistical properties of early-type binary stars. LAMOST provides valuable help in solving this problem.

LAMOST stands for "Large Sky Area Multi-Object Fiber Spectroscopic Telescope", also known as the "Guo Shoujing Telescope". It is an independently innovated telescope with a large field of view, large aperture and the highest spectral acquisition rate developed by the National Astronomical Observatory of the Chinese Academy of Sciences. It can acquire a large amount of optical spectra, providing a rare opportunity for the study of early-type stars.

Dr. Guo Yanjun from the Yunnan Observatory of the Chinese Academy of Sciences and his collaborators grouped a sample of 886 early-type stars with more than six spectral observations in LAMOST DR8 according to their effective temperature, metallicity and projected rotation speed, and conducted a comprehensive correction of the observational data using the Monte Carlo simulation method.

LAMOST and the Galaxy

(Image source: National Astronomical Observatory)

Dr. Guo Yanjun and his collaborators found that the proportion of intrinsic binaries in this sample of early-type stars decreases with decreasing temperature and with decreasing metallicity: the binary proportion of massive O/B stars can reach 76%, while the binary proportion of relatively small B/A stars is about 48%; the binary proportion of early-type stars with solar-like metallicity ([M/H]>-0.1) is 72%, while the binary proportion of early-type stars with poor metallicity ([M/H]<-0.5) is 44%.

At this point, readers may ask: What is the binary ratio? In fact, the binary ratio is an important factor that determines the number and spatial distribution of black hole or neutron star binaries in the universe. The binary ratio given by observation can be used as a lower limit of the theoretical intrinsic binary ratio and constrain theoretical binary models. At the same time, this result can be used as an input parameter for binary star population synthesis, and further understand the formation and evolution of compact binaries.

Editor: Sun Chenyu