Meghnad Saha: Reading the Temperature of Stars

Meghnad Saha was an Indian astrophysicist. He rose from a village shop to the Royal Society.

Where did he come from?

• He was born on 6 October 1893 in the village of Seoratali, near Dacca.
• His father ran a small grocery shop.
• He enrolled in Dhaka Collegiate School in 1905 on a scholarship of Rs 4 a month.
• He boycotted a visit by the governor during the Swadeshi protests. The school expelled him and ended his scholarship.
• A private school in Dhaka then gave him a free place and a stipend.
• In 1909 he came first among students from East Bengal in the entrance exam.

How did he study physics?

He joined Presidency College, Calcutta, in 1911. He took a BSc in 1913 and an MSc in 1915. His classmate S. N. Bose came first in both exams. Saha came second.

What is the memory hook?

Think “1893 born, 1913 BSc, Bose first, Saha second”. Two future stars of physics sat in one class.

Saha was 26 when he worked out his famous result. He was at Calcutta University and had no research supervisor.

What does the equation say?

• Heat strips electrons off atoms. This is called ionization.
• The equation links the ionization of a gas to its temperature and pressure.
• Hotter gas is more ionized. Higher pressure lowers the ionization.

How is it used in astronomy?

A star’s spectrum shows which ions are present. The equation turns that into a temperature. Astronomers can then read a star’s temperature from its light.

What is the memory hook?

Think “temperature and pressure, 1920”. Heat ionizes, pressure resists.

Stars come in different colours and show different spectral lines. Astronomers at Harvard sorted them into classes: O, B, A, F, G, K and M.

What did people first think?

Many assumed that stars of different classes were made of different materials. Saha showed that temperature was the main reason.

What was his calcium example?

• Saha found that calcium becomes fully ionized to Ca+ near 13,000 K.
• Its line fades in stars hotter than the B class.
• So the B stars where it just disappears must be near 13,000 K.

Did Saha write the equation first?

Not quite. Eggert and Lindemann had written the same equation before him. Saha’s great step was to apply it to stars. His first three papers appeared in the Philosophical Magazine between October 1920 and February 1921.

What is the memory hook?

Think “colour changes, composition does not”. Temperature explains the spectral classes.

Saha travelled to Europe in 1920 and worked in Alfred Fowler’s laboratory at Imperial College, London. Honours followed.

What are the main steps?

• He returned to India and taught at Allahabad University.
• He built a furnace there to test thermal ionization in the laboratory.
• In 1927 the Royal Society elected him a Fellow.
• He was the fourth Indian to become a Fellow, after Ramanujan, J. C. Bose and C. V. Raman.
• He later returned to Calcutta as Palit Professor of Physics.

Did he win a Nobel Prize?

No. He was nominated for the Nobel Prize and was shortlisted in 1930. No astrophysicist had yet won it. Hans Bethe was the first, in 1967.

What did he do in politics?

He won a Lok Sabha seat in India’s first general election of 1951-52. He served as a Member of Parliament until his death.

What is the memory hook?

Think “27 FRS, fourth Indian”. Ramanujan, J. C. Bose, Raman, then Saha.

Saha believed that science must serve the nation. He built places where research could grow.

What did he found?

• He founded and edited the journal Science and Culture.
• He pressed for science to guide public policy.
• The Institute of Nuclear Physics was founded in Calcutta in 1949.
• Syama Prasad Mookerjee laid its foundation stone.

When did it open?

Madame Irène Joliot-Curie formally inaugurated the institute on 11 January 1950. That date is now its Foundation Day. After Saha’s death it was renamed the Saha Institute of Nuclear Physics.

What is the memory hook?

Think “founded 1949, opened 1950”. The name came after his death.

Indian calendars disagreed with the actual motions of the sun and moon. After independence the government decided to reform them.

What happened?

• In 1952 the Government of India appointed the Calendar Reform Committee.
• Saha was its chairman.
• The committee submitted its report in 1955.
• The reformed calendar began on 22 March 1957.

What is the new calendar like?

• Its year length and leap year rules match the Gregorian calendar.
• The year count follows the Saka era.
• New Year’s Day falls on the vernal equinox, around 21 or 22 March.
• 22 March 1957 was the start of Saka year 1879.

How did Saha die?

Saha died on 16 February 1956 in New Delhi, aged 62. He collapsed on his way to the Planning Commission office.

What is the memory hook?

Think “committee 1952, calendar 1957”. Saha did not live to see it begin.

Four mix-ups come up again and again. Learn each pair.

What are the traps?

• Saha or Saka? The Saha equation is about stars. The Saka era is the calendar’s year count.
• Saha or Bose? Saha came second in the class. S. N. Bose came first.
• Who wrote it first? Eggert and Lindemann wrote the equation before Saha. Saha applied it to stars.
• Nobel Prize? Saha was nominated but never won.

How do you remember them?

Think “Saha reads stars, Saka counts years”. One name, two different jobs.