The brightest star in the day sky – Our Closest Neighbor Part I

Space has for quite some time now, been the barometer of our civilization’s advancement. The nature of the cosmos itself is such that it makes us wonder in awe, question, contemplate and seek, seek the answers to our beginnings, our end and our relationship with the rest of the universe. But, all of that begins by understanding our home planet, our home star, our solar system and moving beyond. Alpha Centauri is our nearest stellar neighbor. Read on to get introduced to this fascinating world, which might be our first stop on our interstellar sojourn.

Part 2: Alpha Centauri – As observed in nature and culture

Part 3: Discovery and further study of Alpha Centauri

Part 4: The Proxima Puzzle, Stellar Proximities, and Earthly Purview

Our home star, the Sun isn’t an average star. It is unique. No, not because we orbit the Sun but of its special features. What are those? Also, what is it like to be a star? Read on to learn more about the Sun and stars in general. This is the first part of the series on Alpha Centauri.

So, what’s a star? An object that is dense at its center and hot enough to start hydrogen nuclear fusion reactions can be called a star. It is a giant sphere of hydrogen and helium gas at very high temperature. The size of a star is dependent on its age and rate of nuclear fusion reaction going on in its core which produces energy.

Our Home Star

We know our Sun as an average-sized star. Stars, like humans, come in all sizes and colors, with the mass and temperature determined by the rate of energy output. But based on extensive studies, in the early 20th century, scientists declared that the Sun is actually a ‘yellow dwarf star’. The 1921 edition of Encyclopedia Britannica described the Sun as being one of the smallest and faintest stars out there. It looks bright and huge only because of its closeness to us (or our closeness to the Sun!). Later on, our home star was described more like how we know it today – ‘a medium sized, mid-life, commas class star’. But his description fits it best, in comparison to the rest of the observable stars and is a sign of its placement in the HR diagram.

The Sun is special. Not just because of our spatial association but in its composition and nature as well. While in one comparison, the Sun has to be a young star with high metal composition, on the other it must be an old star with low mass and brightness. The reason is the following: In relation to a common star in its stellar neighborhood, the Sun is 10 times heavier, 10 times bigger, twice as hot and 10,000 times brighter! The Sun also has a relatively high metal composition. In the near regions of space, a star the size of Sun is mostly found in a binary system, like Alpha Centauri. Also, it is a non-typical star within the Milky Way galaxy itself, based on its composition. Not to forget, it is part of a small number of stars that have planets orbiting within a habitable zone. In effect, there is a 99.99% chance that you can’t find a star like ours.

Stars come in all sizes (Well, almost all sizes)

There are limits for how big or small stars can be. If a star weighs less than 0.08 times that of the Sun, it would either be a brown dwarf or a big planet (like Jupiter). When a star has a mass more than 100 times the size of the Sun, the need to keep a standard rate of fusion to radiate across such a massive surface gets the better of the star. So smaller stars (< 0.08 times the mass of the Sun) cannot reach the minimum surface temperature to have sustainable life as a normal star while those with bigger mass (> 100 times the mass of the Sun) will have such a high radiation pressure, that it ultimately blocks any addition of mass (and starves the star). It should also be noted that brightness of a star is proportional to its mass. Also, a star operates on a thin boundary between collapsing by gravity from outside or by dispersion caused by pressure from the inside. This is best explained by the ‘Two-component picture of stellar structure’ provided by Arthur Eddington.

Know your Sun

The Sun is made up of 70% hydrogen, 28% helium and 2% of other elements (oxygen, carbon, nitrogen, zirconium, uranium etc.). The nuclear fusion process takes place in the inner region which is about 1/3rd of the size of the Sun. The outer 1/3rd of the Sun is the convection zone. Along with radiation, energy can be transferred in a star also through convection. The combination of Sun’s rotation and convection produces its magnetic field.

The fusion reactions involving hydrogen produce energy at the core that balances the energy that is lost to space through the surface. With a sufficiently hot central region, the Sun is able to create a pressure gradient. In this way, the pressure is maximum at the core with a fall in pressure as we move towards the surface. Gravity pushes the many layers of the Sun towards the core. By supporting them through building enough pressure, the Sun is able to maintain a dynamically stable state. This remains same for other stars as well.

Different regions of the Sun rotate at different speeds. At the equator, it takes 25 earth days to complete one rotation while at the extreme Polar Regions, it takes 35 days. Sunspots are caused by the differential motion of the Sun and are associated with regions of high magnetic activity within the Sun. When they appear in pairs, sunspots exhibit opposite polarity.

Gustave Spörer of Germany developed the law that explains the movement of Sunspots. Spörer’s law states that at the beginning of each solar cycle, the sunspots appear at mid-latitudes and start moving towards the center. Initially, there are fewer sunspots. As they increase, they also move closer to the equatorial region, at which point a new cycle begins. Solar flares are caused by a sudden spurt in magnetic activity. This causes a high-velocity stream of charged particles that is directed towards the solar system. Discovered by Richard Carrington in 1859, solar flares are normally associated with active sunspots. A coronal mass ejection happens when very high volumes of energy are released, in which case a lot of material is discharged into the Solar system at high speeds. Upon interaction with Earth’s atmosphere at the poles, a vivid display of Aurora can be observed.

Some little-known facts about the Sun

  • Between 1645 and 1715, there were no sunspots or solar flares that were observed. This period is called ‘Maunder minimum’. This coincided with the ‘Little Ice Age’ in northern Europe with a drop in temperature across the globe.
  • The Sun rings. Yes, it really does. Portions of the photosphere move upwards while some move downwards. This vertical oscillation propagates through sound waves, which are caused by the pressure exerted by the interior of the Sun. The study of the same feature has a name – ‘Helioseismology’. Astroseismology, the study of similar pulses in stars, help in understanding the interior make-up of a star and a whole lot more.
  • The Sun orbits around the galactic center, once every 260 million years. The path, however, isn’t on a fixed plane, with the path actually resembling a carousel. Sun moves above and below the galactic plane with the same taking a time period of about 30 million years.
  • The Sun is part of the ‘Orion spur’, which happens to be a spiral arm feature of the Milky Way galaxy. Among the stars in the vicinity of our Sun, HD 140283 is the oldest star. Having already been around for approximately 14.5 billion years, it may be one of the first stars to have formed in this universe!

Notes:

Beech, M. (2015).Alpha Centauri. 1st ed. Springer International Publishing. [Minor planet (12343) Martin Beech has been named in recognition of his contributions to meteor physics.]

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  • The nomenclature used properly should be (α – Alpha, ß – Beta). It must be noted that the use of actual words than Greek letters is aimed to make reading easier.
  • AU (Astronomical Unit) – A unit of length effectively equal to the average, or mean, the distance between earth and the Sun, defined as 149,597,870.7 km (92,955,807.3 miles). Source: Encyclopedia Britannica
  • Hertzsprung–Russell diagram is a graph in which the absolute magnitudes (intrinsic brightness) of stars are plotted against their spectral types. (Source: Brittanica.com)
  • The minimum temperature necessary for the nuclear fusion reaction to happen in a star is 10 million Kelvin.
  • The brightest star (apart from the Sun) that is visible to the naked eye is Sirius (meaning ‘Scorching’ in Greek). It is part of the constellation Canis Major (The Great Dog).
  • There are many nomenclature systems, like the Bayer identification (Sirius is Alpha Canoris Majoris), Henry Draper catalog (Sirius is HD 48915) and the Hipparchos data catalog (Sirius is HIP 32349). If the ‘star’ turns out to be a star system, then the component stars are named with letters A, B and so on.
  • Sirius is actually a binary system, with the brighter Sirius A and its white dwarf companion Sirius B. Sirius system is also called Sirius AB.
  • Planets are named after its parent star using a nomenclature that starts with lower letter b. Thus, a planet around Sirius A will be termed as Sirius Ab.

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