Introduction:
- Scientists at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, have recently achieved the most precise determination of the expansion rate of the universe using Mira variable stars.
- This work enhances our understanding of the Hubble Constant, a fundamental parameter in cosmology.
About Mira Stars:
- Mira, also known as Omicron Ceti, is a red giant star located about 300 light-years away in the constellation Cetus.
- The name Mira means “the wonderful” in Latin.
- It became the first variable star ever discovered (observed in 1596 by David Fabricius, studied further in the 17th century).
- Mira stars are typically 200–400 times larger than the Sun but much cooler, with a surface temperature of about 3,000 Kelvin (Sun: ~6,000 K).
Characteristics of Mira Variables:
- They are pulsating red giants in the late stages of stellar evolution.
- Brightness variation cycle: 100 to 1,000 days.
- They can change brightness by a factor of 10 to 1,000 times during each pulsation cycle.
- Mass loss through stellar winds: up to 10⁻⁷ solar masses per year, enriching the interstellar medium.
Astronomical Importance:
- Mira variables follow a clear Period–Luminosity (P–L) relationship, similar to Cepheid variables but cooler and older.
- This makes them reliable standard candles for measuring cosmic distances up to several million light-years.
- Standard candles are crucial to build the Extragalactic Distance Ladder, used to estimate distances beyond the Milky Way.
Recent Development by IUCAA:
- IUCAA scientists calibrated Mira stars with unprecedented accuracy using infrared observations, where they shine brightest.
- Their study allowed a refined measurement of the Hubble Constant (H₀), which describes how fast the universe is expanding.
- This is important in the context of the ongoing “Hubble Tension” – a discrepancy between H₀ values derived from the early universe (Cosmic Microwave Background, ~67 km/s/Mpc) and the late universe (Cepheids/Supernovae, ~73 km/s/Mpc).
- Mira stars provide an independent method to test and potentially resolve this tension.
Global Relevance:
- Adds another rung to the cosmic distance measurement toolkit.
- Helps refine models of dark energy, galaxy evolution, and the fate of the universe.
- Complements other standard candles like Cepheids and Type Supernovae.