Space Physics - Stars and the Universe

Stars are formed from interstellar clouds of dust and gas called nebulae. Gravity pulls the matter together to form a protostar, which increases in temperature until nuclear fusion of hydrogen into helium begins ($4 \, ^{1}_{1}\text{H} \rightarrow \, ^{4}_{2}\text{He} + 2 \, ^{0}_{+1}e + \text{energy}$), marking the start of the Main Sequence phase.

The stable period of a star (Main Sequence) is a result of the balance between the inward force of gravity and the outward pressure from nuclear fusion (hydrostatic equilibrium).

Stars with a mass similar to the Sun eventually evolve into Red Giants, then shed their outer layers to leave behind a hot, dense core called a White Dwarf.

Stars with much higher mass than the Sun evolve into Red Supergiants, which eventually explode in a Supernova, leaving behind either a Neutron Star or a Black Hole.

The orbital speed $v$ of a planet or satellite in a circular orbit of radius $r$ is calculated using the period $T$: $v = \frac{2\pi r}{T}$.

Redshift is the observed increase in the wavelength of light from distant galaxies. The change in wavelength $\Delta \lambda$ relative to the laboratory wavelength $\lambda_0$ is proportional to the recessional velocity $v$ of the galaxy: $\frac{\Delta \lambda}{\lambda_0} \approx \frac{v}{c}$.

Hubble’s Law states that the recessional velocity $v$ of a galaxy is proportional to its distance $d$ from Earth, expressed as $v = H_0 d$, where $H_0$ is the Hubble constant.

The age of the universe can be estimated using the reciprocal of the Hubble constant: $t \approx \frac{1}{H_0}$. Current estimates put this at approximately $13.8$ billion years ($4.35 \times 10^{17} \text{ s}$).