Stellar properties and the Hertzsprung-Russell diagram IB DP Physics Study Notes - 2025 Syllabus
Stellar properties and the Hertzsprung-Russell diagram IB DP Physics Study Notes
Stellar properties and the Hertzsprung-Russell diagram IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
the main regions of the Hertzsprung–Russell (HR) diagram and how to describe the main properties of stars in these regions
the use of stellar parallax as a method to determine the distance d to celestial bodies as given by
d(parsec) = 1/ p(arc‐second)how to determine stellar radii.
Standard level and higher level: 4 hours
Additional higher level: There is no additional higher level content.
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
What is a Parsec? (pc)
- A unit of distance that we use for determining the distance of stars
- It is calculated using the diagram here
- It is defined as the length of the adjacent side of an imaginary right triangle in space.
- The two dimensions that specify this triangle are the parallax angle (defined as 1 arcsecond or 1’’) and the opposite side (which is defined as 1 astronomical unit (AU), the distance from the Earth to the Sun).
- Given these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.
Parsec and Friends
Stellar Parallax
- The apparent shifting of a distant object against a stationary background when viewed from two different perspectives
- Think about looking out of a window with a plant on the sill. As you walk past the window the object will appear to move relative to the background outside.
Parallax
- The shift is very slight relative to the “fixed” stars behind the observed star but sufficient for distant stars… but not too distant (100pc or less).
- Formula: $d=\frac{1}{p}$, where d is the distance to the star and p is the parallax angle.
Limitations of the Parallax method
- Stars that are too far away will not show enough parallax to calculate their distance
- Viewing stars from a space-based position helps to eliminate some of the error and increase the range, but at some point the error in the system equals or exceeds the angle being measured
- Beyond that, other methods can be used
Atomic Spectra and You!
- Passing a sufficiently high potential through a tube of a specific gas will cause it to glow, as we see in the fluorescent lights in the classroom
- Different elements produce different discrete spectra, unlike an incandescent light source which produces a continuous range of colors
Emission/Absorption
If we shine light from an incandescent source through a tube made of a particular gas, only the wavelengths that would be emitted by that gas are absorbed
Absorption Spectra
Thus, we can determine the surface elements of a star based on the absorption spectrum that we see when we look at that star
Spectral Classification
- Stars with similar spectra are grouped together into spectral classes, which relates directly to their surface temperatures.
- Temperature influences a star’s ability to ionize or excite the atoms of its surface elements, thus impacting the spectrum we see
- Regardless of class, all stars are essentially 71% H, 27% He, and 1% other
Wien’s Displacement Law
- Wien discovered an empirical relationship between the maximum value of the wavelength emitted by a black body and its temperature which can be stated as:
- This law applies to the spectra of stars as well, which means that we can use Wien’s Law to find the temperature of a star based on the spectra of radiation that it emits
- Given that we can measure find a star’s luminosity as well, we can actually use the S-B Law for the more practical purpose of determining the radius of a star.
IB Physics Stellar properties and the Hertzsprung-Russell diagram Exam Style Worked Out Questions
Question
The Hertzsprung-Russell diagram shows two stars, $\mathrm{X}$ and $\mathrm{Y}$.
What is $\frac{\text { radius of } X}{\text { radius of } Y}$ ?
A. $\frac{1}{16}$
B. $\frac{1}{4}$
C. 4
D. 16
▶️Answer/Explanation
Ans:A