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--- courses:ast403:extragalactic-distance-scale [2026/02/12 09:03] – shuvo
+++ courses:ast403:extragalactic-distance-scale [2026/02/12 09:05] (current) – shuvo
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 ===== Extragalactic Distance Scales =====
-Extragalactic distance determination relies on a **distance ladder** where absolute distances to nearby objects are used to calibrate relative indicators that reach further into the Universe. This calibration is essential to determine the **Hubble constant** ($H_0$), as redshift-based distances are only accurate if $H_0$ is known and peculiar velocities (local gravitational motions) are negligible.
+Extragalactic distance determination relies on a distance ladder where absolute distances to nearby objects are used to calibrate relative indicators that reach further into the Universe. This calibration is essential to determine the Hubble constant ($H_0$), as redshift-based distances are only accurate if $H_0$ is known and peculiar velocities (local gravitational motions) are negligible.
   * **Primary Distance Indicators:**
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 Primary indicators are used to establish the first rungs of the ladder, often focusing on the Large Magellanic Cloud (LMC).
-//Geometric Method (SN 1987A): One of the most precise methods involves the ring around supernova SN 1987A. By comparing the time delay between the illumination of the nearest and farthest parts of the ring with its angular diameter (~1.7"), astronomers derive a physical diameter and a distance of $D_{SN1987A} \approx 51.8 \text{ kpc}$.
+//Geometric Method (SN 1987A):// One of the most precise methods involves the ring around supernova SN 1987A. By comparing the time delay between the illumination of the nearest and farthest parts of the ring with its angular diameter (~1.7"), astronomers derive a physical diameter and a distance of $D_{SN1987A} \approx 51.8 \text{ kpc}$.
-//Cepheid Variables:** These young stars follow a well-defined period–luminosity (PL) relation, where their intrinsic luminosity ($L$) is related to their pulsation period ($P$): $P \propto L^{7/12}$. Calibrated in the LMC, Cepheids are visible with the Hubble Space Telescope (HST) out to the Virgo Cluster (~16 Mpc).
+//Cepheid Variables:// These young stars follow a well-defined period–luminosity (PL) relation, where their intrinsic luminosity ($L$) is related to their pulsation period ($P$): $P \propto L^{7/12}$. Calibrated in the LMC, Cepheids are visible with the Hubble Space Telescope (HST) out to the Virgo Cluster (~16 Mpc).
-//RR Lyrae Stars:** These Population II stars are found in globular clusters and the Galactic bulge. Their absolute visual magnitudes are nearly constant ($M_V \approx 0.6$), though more precise estimates account for metallicity:
+//RR Lyrae Stars:// These Population II stars are found in globular clusters and the Galactic bulge. Their absolute visual magnitudes are nearly constant ($M_V \approx 0.6$), though more precise estimates account for metallicity: